diff --git a/pomodoro_app/android/app/src/main/AndroidManifest.xml b/pomodoro_app/android/app/src/main/AndroidManifest.xml
index 05764e4..51d5919 100644
--- a/pomodoro_app/android/app/src/main/AndroidManifest.xml
+++ b/pomodoro_app/android/app/src/main/AndroidManifest.xml
@@ -3,6 +3,7 @@
     <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
     <uses-permission android:name="android.permission.CHANGE_WIFI_MULTICAST_STATE"/>
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <application
         android:label="pomodoro_app"
         android:name="${applicationName}"
diff --git a/pomodoro_app/lib/main.dart b/pomodoro_app/lib/main.dart
new file mode 100644
index 0000000..dfc71b7
--- /dev/null
+++ b/pomodoro_app/lib/main.dart
@@ -0,0 +1,24 @@
+import 'package:flutter/material.dart';
+
+import 'screens/pomodoro_screen.dart';
+import 'theme/pomodoro_theme.dart';
+
+void main() {
+  runApp(const PomodoroApp());
+}
+
+/// The root widget of the Pomodoro application.
+class PomodoroApp extends StatelessWidget {
+  /// Creates a [PomodoroApp].
+  const PomodoroApp({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return MaterialApp(
+      title: 'Pomodoro',
+      debugShowCheckedModeBanner: false,
+      theme: PomodoroTheme.darkTheme,
+      home: const PomodoroScreen(),
+    );
+  }
+}
diff --git a/pomodoro_app/lib/models/pomodoro_state.dart b/pomodoro_app/lib/models/pomodoro_state.dart
new file mode 100644
index 0000000..450a0fa
--- /dev/null
+++ b/pomodoro_app/lib/models/pomodoro_state.dart
@@ -0,0 +1,205 @@
+/// Defines the timer style (technique) the user can choose.
+enum TimerStyle {
+  /// Classic Pomodoro: 25 min work, 5 min short break, 15 min long break.
+  pomodoro,
+
+  /// Ultraradian rhythm: 90 min work, 30 min break.
+  ultraradian,
+}
+
+/// Extension on [TimerStyle] to provide display labels and default durations.
+extension TimerStyleConfig on TimerStyle {
+  /// Human-readable label for the timer style.
+  String get label {
+    switch (this) {
+      case TimerStyle.pomodoro:
+        return 'Pomodoro';
+      case TimerStyle.ultraradian:
+        return 'Ultraradian';
+    }
+  }
+
+  /// Default work duration in minutes.
+  int get defaultWorkMinutes {
+    switch (this) {
+      case TimerStyle.pomodoro:
+        return 25;
+      case TimerStyle.ultraradian:
+        return 90;
+    }
+  }
+
+  /// Default short break duration in minutes.
+  int get defaultShortBreakMinutes {
+    switch (this) {
+      case TimerStyle.pomodoro:
+        return 5;
+      case TimerStyle.ultraradian:
+        return 30;
+    }
+  }
+
+  /// Default long break duration in minutes.
+  int get defaultLongBreakMinutes {
+    switch (this) {
+      case TimerStyle.pomodoro:
+        return 15;
+      case TimerStyle.ultraradian:
+        return 30;
+    }
+  }
+
+  /// Default number of work sessions before a long break.
+  int get defaultPomodorosPerCycle {
+    switch (this) {
+      case TimerStyle.pomodoro:
+        return 4;
+      case TimerStyle.ultraradian:
+        return 1;
+    }
+  }
+}
+
+/// Defines the different modes of a Pomodoro session.
+enum PomodoroMode {
+  /// A work session (default 25 minutes).
+  work,
+
+  /// A short break between work sessions (default 5 minutes).
+  shortBreak,
+
+  /// A long break after completing a cycle (default 15 minutes).
+  longBreak,
+}
+
+/// Extension on [PomodoroMode] to provide display labels.
+extension PomodoroModeLabel on PomodoroMode {
+  /// Human-readable label for the mode.
+  String get label {
+    switch (this) {
+      case PomodoroMode.work:
+        return 'Work';
+      case PomodoroMode.shortBreak:
+        return 'Short Break';
+      case PomodoroMode.longBreak:
+        return 'Long Break';
+    }
+  }
+}
+
+/// Immutable snapshot of the Pomodoro timer state.
+class PomodoroState {
+  /// Creates a [PomodoroState].
+  const PomodoroState({
+    required this.mode,
+    required this.remainingSeconds,
+    required this.totalSeconds,
+    required this.isRunning,
+    required this.completedPomodoros,
+    required this.pomodorosPerCycle,
+  });
+
+  /// Creates the default initial state.
+  factory PomodoroState.initial({
+    int workMinutes = 25,
+    int shortBreakMinutes = 5,
+    int longBreakMinutes = 15,
+    int pomodorosPerCycle = 4,
+  }) {
+    final totalSeconds = workMinutes * 60;
+    return PomodoroState(
+      mode: PomodoroMode.work,
+      remainingSeconds: totalSeconds,
+      totalSeconds: totalSeconds,
+      isRunning: false,
+      completedPomodoros: 0,
+      pomodorosPerCycle: pomodorosPerCycle,
+    );
+  }
+
+  /// The current timer mode.
+  final PomodoroMode mode;
+
+  /// Seconds left on the current timer.
+  final int remainingSeconds;
+
+  /// Total seconds for the current mode.
+  final int totalSeconds;
+
+  /// Whether the timer is currently running.
+  final bool isRunning;
+
+  /// Number of completed work sessions in the current cycle.
+  final int completedPomodoros;
+
+  /// Number of pomodoros before a long break.
+  final int pomodorosPerCycle;
+
+  /// Progress as a value between 0.0 and 1.0.
+  double get progress {
+    if (totalSeconds == 0) return 1.0;
+    return 1.0 - (remainingSeconds / totalSeconds);
+  }
+
+  /// Display label for the current mode, context-aware.
+  ///
+  /// When [pomodorosPerCycle] is 1 (e.g. ultraradian), breaks are simply
+  /// labelled "Break" instead of "Short Break" or "Long Break".
+  String get modeDisplayLabel {
+    if (pomodorosPerCycle <= 1 && mode != PomodoroMode.work) {
+      return 'Break';
+    }
+    return mode.label;
+  }
+
+  /// Formatted time string (MM:SS).
+  String get formattedTime {
+    final minutes = remainingSeconds ~/ 60;
+    final seconds = remainingSeconds % 60;
+    return '${minutes.toString().padLeft(2, '0')}:'
+        '${seconds.toString().padLeft(2, '0')}';
+  }
+
+  /// Creates a copy with the given fields replaced.
+  PomodoroState copyWith({
+    PomodoroMode? mode,
+    int? remainingSeconds,
+    int? totalSeconds,
+    bool? isRunning,
+    int? completedPomodoros,
+    int? pomodorosPerCycle,
+  }) {
+    return PomodoroState(
+      mode: mode ?? this.mode,
+      remainingSeconds: remainingSeconds ?? this.remainingSeconds,
+      totalSeconds: totalSeconds ?? this.totalSeconds,
+      isRunning: isRunning ?? this.isRunning,
+      completedPomodoros: completedPomodoros ?? this.completedPomodoros,
+      pomodorosPerCycle: pomodorosPerCycle ?? this.pomodorosPerCycle,
+    );
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (identical(this, other)) return true;
+    return other is PomodoroState &&
+        other.mode == mode &&
+        other.remainingSeconds == remainingSeconds &&
+        other.totalSeconds == totalSeconds &&
+        other.isRunning == isRunning &&
+        other.completedPomodoros == completedPomodoros &&
+        other.pomodorosPerCycle == pomodorosPerCycle;
+  }
+
+  @override
+  int get hashCode {
+    return Object.hash(
+      mode,
+      remainingSeconds,
+      totalSeconds,
+      isRunning,
+      completedPomodoros,
+      pomodorosPerCycle,
+    );
+  }
+}
diff --git a/pomodoro_app/lib/screens/pomodoro_screen.dart b/pomodoro_app/lib/screens/pomodoro_screen.dart
new file mode 100644
index 0000000..828f0a0
--- /dev/null
+++ b/pomodoro_app/lib/screens/pomodoro_screen.dart
@@ -0,0 +1,167 @@
+import 'package:flutter/material.dart';
+
+import '../models/pomodoro_state.dart';
+import '../services/pomodoro_timer.dart';
+import '../services/notification_service.dart';
+import '../services/sound_service.dart';
+import '../services/sync_service.dart';
+import '../widgets/pomodoro_indicators.dart';
+import '../widgets/timer_controls.dart';
+import '../widgets/timer_display.dart';
+
+/// The main screen of the Pomodoro app.
+///
+/// Displays the timer, controls, and session indicators in a responsive
+/// layout that works on both mobile and desktop.
+class PomodoroScreen extends StatefulWidget {
+  /// Creates a [PomodoroScreen].
+  const PomodoroScreen({this.timer, this.syncService, super.key});
+
+  /// Optional timer instance for testing. If null, creates a default one.
+  final PomodoroTimer? timer;
+
+  /// Optional sync service for testing. If null, creates a default one.
+  final SyncService? syncService;
+
+  @override
+  State<PomodoroScreen> createState() => PomodoroScreenState();
+}
+
+/// State for [PomodoroScreen], exposed for testing.
+@visibleForTesting
+class PomodoroScreenState extends State<PomodoroScreen> {
+  PomodoroTimer? _timer;
+  SyncService? _syncService;
+  bool _ownsTimer = false;
+  bool _ownsSyncService = false;
+  bool _initialized = false;
+
+  @override
+  void initState() {
+    super.initState();
+
+    if (widget.timer != null) {
+      // Test path: synchronous init, no sync service needed.
+      _timer = widget.timer!;
+      _syncService = widget.syncService;
+      _timer!.addListener(_onTimerChanged);
+      _initialized = true;
+    } else {
+      // Production path: async init with sync service.
+      _initAsync();
+    }
+  }
+
+  Future<void> _initAsync() async {
+    _syncService = SyncService(
+      onStateReceived: _onRemoteState,
+    );
+    _ownsSyncService = true;
+    await _syncService!.start();
+
+    _timer = PomodoroTimer(
+      syncService: _syncService,
+      soundService: SoundService(),
+      notificationService: NotificationService(),
+    );
+    _ownsTimer = true;
+
+    _timer!.addListener(_onTimerChanged);
+    _initialized = true;
+    if (mounted) setState(() {});
+  }
+
+  void _onRemoteState(PomodoroState state, String action) {
+    _timer?.applyRemoteState(state, action);
+  }
+
+  void _onTimerChanged() {
+    if (mounted) setState(() {});
+  }
+
+  @override
+  void dispose() {
+    _timer?.removeListener(_onTimerChanged);
+    if (_ownsTimer) _timer?.dispose();
+    if (_ownsSyncService) _syncService?.dispose();
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    if (!_initialized || _timer == null) {
+      return const Scaffold(
+        body: Center(child: CircularProgressIndicator()),
+      );
+    }
+
+    final timer = _timer!;
+    final state = timer.state;
+
+    return Scaffold(
+      body: SafeArea(
+        child: Center(
+          child: ConstrainedBox(
+            constraints: const BoxConstraints(maxWidth: 500),
+            child: Padding(
+              padding: const EdgeInsets.symmetric(horizontal: 24),
+              child: Column(
+                mainAxisAlignment: MainAxisAlignment.center,
+                children: [
+                  const Spacer(flex: 2),
+                  // Timer style picker.
+                  SegmentedButton<TimerStyle>(
+                    segments: const [
+                      ButtonSegment(
+                        value: TimerStyle.pomodoro,
+                        label: Text('Pomodoro'),
+                        icon: Icon(Icons.timer),
+                      ),
+                      ButtonSegment(
+                        value: TimerStyle.ultraradian,
+                        label: Text('Ultraradian'),
+                        icon: Icon(Icons.self_improvement),
+                      ),
+                    ],
+                    selected: {timer.timerStyle},
+                    onSelectionChanged: (selected) {
+                      timer.switchStyle(selected.first);
+                    },
+                  ),
+                  const SizedBox(height: 16),
+                  // Timer display.
+                  Expanded(
+                    flex: 5,
+                    child: TimerDisplay(state: state),
+                  ),
+                  const SizedBox(height: 32),
+                  // Controls.
+                  TimerControls(
+                    state: state,
+                    onStart: timer.start,
+                    onPause: timer.pause,
+                    onReset: timer.reset,
+                    onSkip: timer.skip,
+                  ),
+                  const SizedBox(height: 32),
+                  // Session indicators.
+                  PomodoroIndicators(state: state),
+                  const SizedBox(height: 16),
+                  // Completed count.
+                  Text(
+                    '${state.completedPomodoros} '
+                    '${timer.timerStyle.label.toLowerCase()}'
+                    '${state.completedPomodoros == 1 ? '' : 's'}'
+                    ' completed',
+                    style: Theme.of(context).textTheme.bodyLarge,
+                  ),
+                  const Spacer(flex: 2),
+                ],
+              ),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}
diff --git a/pomodoro_app/lib/services/notification_service.dart b/pomodoro_app/lib/services/notification_service.dart
new file mode 100644
index 0000000..2476c8a
--- /dev/null
+++ b/pomodoro_app/lib/services/notification_service.dart
@@ -0,0 +1,155 @@
+import 'dart:io';
+
+import 'package:flutter/foundation.dart';
+
+import '../models/pomodoro_state.dart';
+
+/// Sends desktop notifications showing Pomodoro timer status.
+///
+/// Uses the freedesktop D-Bus Notifications interface via `gdbus` to show,
+/// update, and dismiss notifications. The notification includes the current
+/// mode, remaining time, and a progress bar. Action buttons (Pause / Skip /
+/// Start) are displayed for quick interaction.
+class NotificationService {
+  /// Creates a [NotificationService].
+  ///
+  /// Pass a custom [runProcess] for testing.
+  NotificationService({
+    @visibleForTesting
+    Future<ProcessResult> Function(String, List<String>)? runProcess,
+  }) : _runProcess = runProcess ?? Process.run;
+
+  final Future<ProcessResult> Function(String, List<String>) _runProcess;
+  int _currentId = 0;
+  bool _disposed = false;
+
+  static const _dbusDest = 'org.freedesktop.Notifications';
+  static const _dbusPath = '/org/freedesktop/Notifications';
+
+  /// The notification ID currently shown (0 means none).
+  @visibleForTesting
+  int get currentId => _currentId;
+
+  /// Shows or updates the timer notification with the current [state].
+  ///
+  /// The notification replaces any previous one so only a single
+  /// notification is visible at a time.
+  Future<void> showTimer({required PomodoroState state}) async {
+    if (_disposed) return;
+
+    final title = '${state.mode.label} \u2013 ${state.formattedTime}';
+    final body = _progressBar(state.progress);
+
+    await _notify(
+      title: title,
+      body: body,
+      actions: state.isRunning
+          ? ['pause', 'Pause', 'skip', 'Skip']
+          : ['start', 'Start'],
+    );
+  }
+
+  /// Shows a notification that the session has completed.
+  Future<void> showSessionComplete({
+    required PomodoroMode completedMode,
+    required PomodoroMode nextMode,
+  }) async {
+    if (_disposed) return;
+
+    final title = '${completedMode.label} complete!';
+    final body = 'Up next: ${nextMode.label}';
+
+    await _notify(title: title, body: body, actions: ['start', 'Start']);
+  }
+
+  /// Cancels the currently shown notification.
+  Future<void> cancel() async {
+    if (_disposed || _currentId == 0) return;
+
+    try {
+      await _runProcess('gdbus', [
+        'call',
+        '--session',
+        '--dest',
+        _dbusDest,
+        '--object-path',
+        _dbusPath,
+        '--method',
+        'org.freedesktop.Notifications.CloseNotification',
+        '$_currentId',
+      ]);
+    } on Object catch (e) {
+      debugPrint('NotificationService: Close error: $e');
+    }
+    _currentId = 0;
+  }
+
+  /// Releases resources. Does not await the underlying cancel.
+  void dispose() {
+    if (_disposed) return;
+    if (_currentId != 0) {
+      // Fire-and-forget; the notification daemon cleans up on exit.
+      unawaited(cancel());
+    }
+    _disposed = true;
+  }
+
+  // ------------------------------------------------------------------
+  // Private helpers
+  // ------------------------------------------------------------------
+
+  Future<void> _notify({
+    required String title,
+    required String body,
+    List<String> actions = const [],
+  }) async {
+    final actionsStr = actions.isEmpty
+        ? '[]'
+        : '[${actions.map((a) => "'$a'").join(', ')}]';
+
+    try {
+      final result = await _runProcess('gdbus', [
+        'call',
+        '--session',
+        '--dest',
+        _dbusDest,
+        '--object-path',
+        _dbusPath,
+        '--method',
+        'org.freedesktop.Notifications.Notify',
+        'Pomodoro',
+        '$_currentId',
+        'appointment-soon',
+        title,
+        body,
+        actionsStr,
+        '{}',
+        '0',
+      ]);
+
+      final match =
+          RegExp(r'\(uint32 (\d+),?\)').firstMatch(result.stdout as String);
+      if (match != null) {
+        _currentId = int.parse(match.group(1)!);
+      }
+    } on Object catch (e) {
+      debugPrint('NotificationService: Notify error: $e');
+    }
+  }
+
+  /// Builds a text-based progress bar for the notification body.
+  @visibleForTesting
+  static String progressBar(double progress) => _progressBar(progress);
+
+  static String _progressBar(double progress) {
+    const total = 20;
+    final filled = (progress * total).round();
+    final empty = total - filled;
+    return '${'█' * filled}${'░' * empty}';
+  }
+}
+
+/// Completes a future without requiring `await`.
+///
+/// Prevents the `unawaited_futures` lint in fire-and-forget calls.
+void unawaited(Future<void> future) {}
diff --git a/pomodoro_app/lib/services/pomodoro_timer.dart b/pomodoro_app/lib/services/pomodoro_timer.dart
new file mode 100644
index 0000000..3008976
--- /dev/null
+++ b/pomodoro_app/lib/services/pomodoro_timer.dart
@@ -0,0 +1,269 @@
+import 'dart:async';
+
+import 'package:flutter/foundation.dart';
+
+import '../models/pomodoro_state.dart';
+import 'notification_service.dart';
+import 'sound_service.dart';
+import 'sync_service.dart';
+
+/// Manages the Pomodoro timer logic, independent of UI framework.
+///
+/// Optionally synchronizes state across devices via [SyncService].
+class PomodoroTimer extends ChangeNotifier {
+  /// Creates a [PomodoroTimer] with configurable durations.
+  PomodoroTimer({
+    int? workMinutes,
+    int? shortBreakMinutes,
+    int? longBreakMinutes,
+    int? pomodorosPerCycle,
+    TimerStyle timerStyle = TimerStyle.pomodoro,
+    this.syncService,
+    SoundService? soundService,
+    NotificationService? notificationService,
+    @visibleForTesting Timer Function(Duration, void Function(Timer))? timerFactory,
+  })  : _timerStyle = timerStyle,
+        _soundService = soundService,
+        _notificationService = notificationService,
+        _timerFactory = timerFactory ?? Timer.periodic {
+    _workMinutes = workMinutes ?? timerStyle.defaultWorkMinutes;
+    _shortBreakMinutes = shortBreakMinutes ?? timerStyle.defaultShortBreakMinutes;
+    _longBreakMinutes = longBreakMinutes ?? timerStyle.defaultLongBreakMinutes;
+    _pomodorosPerCycle = pomodorosPerCycle ?? timerStyle.defaultPomodorosPerCycle;
+    _state = PomodoroState.initial(
+      workMinutes: _workMinutes,
+      shortBreakMinutes: _shortBreakMinutes,
+      longBreakMinutes: _longBreakMinutes,
+      pomodorosPerCycle: _pomodorosPerCycle,
+    );
+  }
+
+  /// Duration of a work session in minutes.
+  late int _workMinutes;
+
+  /// Duration of a short break in minutes.
+  late int _shortBreakMinutes;
+
+  /// Duration of a long break in minutes.
+  late int _longBreakMinutes;
+
+  /// Number of work sessions before a long break.
+  late int _pomodorosPerCycle;
+
+  /// The current timer style.
+  TimerStyle _timerStyle;
+
+  /// Optional sync service for LAN synchronization.
+  final SyncService? syncService;
+
+  final SoundService? _soundService;
+  final NotificationService? _notificationService;
+  final Timer Function(Duration, void Function(Timer)) _timerFactory;
+
+  late PomodoroState _state;
+  Timer? _timer;
+
+  /// Whether we are currently applying a remote state (prevents echo).
+  bool _applyingRemote = false;
+
+  /// The current state of the timer.
+  PomodoroState get state => _state;
+
+  /// The active timer style.
+  TimerStyle get timerStyle => _timerStyle;
+
+  /// Switches to a different timer style, resetting all progress.
+  void switchStyle(TimerStyle style) {
+    if (style == _timerStyle) return;
+    _timer?.cancel();
+    _timer = null;
+    _timerStyle = style;
+    _workMinutes = style.defaultWorkMinutes;
+    _shortBreakMinutes = style.defaultShortBreakMinutes;
+    _longBreakMinutes = style.defaultLongBreakMinutes;
+    _pomodorosPerCycle = style.defaultPomodorosPerCycle;
+    _state = PomodoroState.initial(
+      workMinutes: _workMinutes,
+      shortBreakMinutes: _shortBreakMinutes,
+      longBreakMinutes: _longBreakMinutes,
+      pomodorosPerCycle: _pomodorosPerCycle,
+    );
+    _notificationService?.cancel();
+    notifyListeners();
+    syncService?.stopHeartbeat();
+  }
+
+  /// Starts or resumes the timer.
+  void start() {
+    if (_state.isRunning) return;
+    _state = _state.copyWith(isRunning: true);
+    notifyListeners();
+    _startTicking();
+    _notificationService?.showTimer(state: _state);
+    _broadcastIfLocal('start');
+    syncService?.startHeartbeat(() => _state);
+  }
+
+  /// Pauses the timer.
+  void pause() {
+    if (!_state.isRunning) return;
+    _timer?.cancel();
+    _timer = null;
+    _state = _state.copyWith(isRunning: false);
+    _notificationService?.cancel();
+    notifyListeners();
+    _broadcastIfLocal('pause');
+    syncService?.stopHeartbeat();
+  }
+
+  /// Resets the current session timer without changing the mode.
+  void reset() {
+    _timer?.cancel();
+    _timer = null;
+    _state = _state.copyWith(
+      remainingSeconds: _state.totalSeconds,
+      isRunning: false,
+    );
+    _notificationService?.cancel();
+    notifyListeners();
+    _broadcastIfLocal('reset');
+    syncService?.stopHeartbeat();
+  }
+
+  /// Skips to the next session, treating the current one as completed.
+  void skip() {
+    _timer?.cancel();
+    _timer = null;
+    _onSessionComplete();
+    _broadcastIfLocal('skip');
+    syncService?.stopHeartbeat();
+  }
+
+  /// Applies state received from a remote device via [SyncService].
+  void applyRemoteState(PomodoroState remoteState, String action) {
+    _applyingRemote = true;
+
+    _timer?.cancel();
+    _timer = null;
+
+    _state = remoteState;
+
+    if (_state.isRunning) {
+      _startTicking();
+    }
+
+    notifyListeners();
+    _applyingRemote = false;
+  }
+
+  void _tick(Timer timer) {
+    if (_state.remainingSeconds <= 1) {
+      timer.cancel();
+      _timer = null;
+      _onSessionComplete();
+    } else {
+      _state = _state.copyWith(
+        remainingSeconds: _state.remainingSeconds - 1,
+      );
+      _updateNotification();
+      notifyListeners();
+    }
+  }
+
+  void _onSessionComplete() {
+    if (_state.mode == PomodoroMode.work) {
+      final newCompleted = _state.completedPomodoros + 1;
+      _state = _state.copyWith(
+        completedPomodoros: newCompleted,
+        remainingSeconds: 0,
+        isRunning: false,
+      );
+    } else {
+      _state = _state.copyWith(
+        remainingSeconds: 0,
+        isRunning: false,
+      );
+    }
+    notifyListeners();
+    final completedMode = _state.mode;
+    _advanceToNextMode();
+    _soundService?.playTransitionSound(
+      completedMode: completedMode,
+      nextMode: _state.mode,
+    );
+    _notificationService?.showSessionComplete(
+      completedMode: completedMode,
+      nextMode: _state.mode,
+    );
+    notifyListeners();
+  }
+
+  void _advanceToNextMode() {
+    switch (_state.mode) {
+      case PomodoroMode.work:
+        if (_state.completedPomodoros > 0 &&
+            _state.completedPomodoros % _pomodorosPerCycle == 0) {
+          _setMode(PomodoroMode.longBreak);
+        } else {
+          _setMode(PomodoroMode.shortBreak);
+        }
+      case PomodoroMode.shortBreak:
+      case PomodoroMode.longBreak:
+        _setMode(PomodoroMode.work);
+    }
+  }
+
+  void _setMode(PomodoroMode mode) {
+    final totalSeconds = _durationForMode(mode) * 60;
+    _state = _state.copyWith(
+      mode: mode,
+      remainingSeconds: totalSeconds,
+      totalSeconds: totalSeconds,
+      isRunning: false,
+    );
+  }
+
+  int _durationForMode(PomodoroMode mode) {
+    switch (mode) {
+      case PomodoroMode.work:
+        return _workMinutes;
+      case PomodoroMode.shortBreak:
+        return _shortBreakMinutes;
+      case PomodoroMode.longBreak:
+        return _longBreakMinutes;
+    }
+  }
+
+  void _startTicking() {
+    _timer = _timerFactory(
+      const Duration(seconds: 1),
+      _tick,
+    );
+  }
+
+  /// Broadcasts state to peers only if this is a local user action.
+  void _broadcastIfLocal(String action) {
+    if (!_applyingRemote) {
+      syncService?.broadcast(_state, action);
+    }
+  }
+
+  /// Interval in seconds between notification updates while running.
+  static const _notifyIntervalSeconds = 30;
+
+  void _updateNotification() {
+    if (_notificationService == null) return;
+    if (_state.remainingSeconds % _notifyIntervalSeconds == 0) {
+      _notificationService.showTimer(state: _state);
+    }
+  }
+
+  @override
+  void dispose() {
+    _timer?.cancel();
+    syncService?.stopHeartbeat();
+    _soundService?.dispose();
+    _notificationService?.dispose();
+    super.dispose();
+  }
+}
diff --git a/pomodoro_app/lib/services/sound_service.dart b/pomodoro_app/lib/services/sound_service.dart
new file mode 100644
index 0000000..bf48048
--- /dev/null
+++ b/pomodoro_app/lib/services/sound_service.dart
@@ -0,0 +1,78 @@
+import 'package:audioplayers/audioplayers.dart';
+import 'package:flutter/foundation.dart';
+
+import '../models/pomodoro_state.dart';
+
+/// Plays notification sounds for Pomodoro timer transitions.
+///
+/// Each transition type has a distinct sound:
+/// - Work done → ascending chime
+/// - Short break done → gentle double ping
+/// - Long break starting → descending celebration
+/// - Long break done → rapid wake-up beeps
+class SoundService {
+  /// Creates a [SoundService].
+  ///
+  /// Pass a custom [playCallback] for testing.
+  SoundService({
+    @visibleForTesting Future<void> Function(String assetPath)? playCallback,
+  }) : _playCallback = playCallback;
+
+  final Future<void> Function(String assetPath)? _playCallback;
+  AudioPlayer? _player;
+  bool _disposed = false;
+
+  static const _assetPrefix = 'sounds';
+
+  /// Plays the appropriate sound for a mode transition.
+  ///
+  /// [completedMode] is the mode that just finished.
+  /// [nextMode] is the mode that is starting.
+  Future<void> playTransitionSound({
+    required PomodoroMode completedMode,
+    required PomodoroMode nextMode,
+  }) async {
+    if (_disposed) return;
+
+    final assetPath = _assetForTransition(completedMode, nextMode);
+    if (assetPath == null) return;
+
+    try {
+      if (_playCallback != null) {
+        await _playCallback(assetPath);
+      } else {
+        _player?.dispose();
+        _player = AudioPlayer();
+        await _player!.play(AssetSource('$_assetPrefix/$assetPath'));
+      }
+      debugPrint('SoundService: Playing $assetPath');
+    } on Object catch (e) {
+      debugPrint('SoundService: Playback error: $e');
+    }
+  }
+
+  /// Returns the WAV filename for a given transition, or null if none.
+  static String? _assetForTransition(
+    PomodoroMode completedMode,
+    PomodoroMode nextMode,
+  ) {
+    switch (completedMode) {
+      case PomodoroMode.work:
+        if (nextMode == PomodoroMode.longBreak) {
+          return 'long_break_start.wav';
+        }
+        return 'work_done.wav';
+      case PomodoroMode.shortBreak:
+        return 'short_break_done.wav';
+      case PomodoroMode.longBreak:
+        return 'long_break_done.wav';
+    }
+  }
+
+  /// Releases audio resources.
+  void dispose() {
+    _disposed = true;
+    _player?.dispose();
+    _player = null;
+  }
+}
diff --git a/pomodoro_app/lib/services/sync_service.dart b/pomodoro_app/lib/services/sync_service.dart
new file mode 100644
index 0000000..7c19861
--- /dev/null
+++ b/pomodoro_app/lib/services/sync_service.dart
@@ -0,0 +1,252 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'dart:math';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+
+import '../models/pomodoro_state.dart';
+
+/// Callback type for receiving a synced [PomodoroState] and action name.
+typedef SyncCallback = void Function(PomodoroState state, String action);
+
+/// Provides LAN synchronization between Pomodoro app instances using
+/// UDP broadcast.
+///
+/// Uses subnet broadcast (255.255.255.255) instead of multicast for
+/// maximum compatibility across platforms. A unique [deviceId] prevents
+/// echo (processing own messages).
+class SyncService {
+  /// Creates a [SyncService].
+  ///
+  /// [onStateReceived] is called when a remote device broadcasts a state
+  /// change. [port] can be overridden for testing.
+  SyncService({
+    required this.onStateReceived,
+    this.port = 41234,
+    @visibleForTesting String? deviceId,
+    @visibleForTesting
+    Future<RawDatagramSocket> Function(dynamic host, int port)?
+        socketFactory,
+  })  : deviceId = deviceId ?? _generateDeviceId(),
+        _socketFactory = socketFactory;
+
+  /// Unique identifier for this device instance.
+  final String deviceId;
+
+  /// UDP port for sync messages.
+  final int port;
+
+  /// UDP port for wake signals (separate from sync to allow a daemon to
+  /// listen without conflicting with the app's sync socket).
+  static const int wakePort = 41235;
+
+  /// Called when a state update is received from another device.
+  final SyncCallback onStateReceived;
+
+  final Future<RawDatagramSocket> Function(dynamic host, int port)?
+      _socketFactory;
+
+  RawDatagramSocket? _socket;
+  Timer? _heartbeat;
+  bool _disposed = false;
+
+  static const _methodChannel = MethodChannel('pomodoro_multicast_lock');
+
+  /// Whether the service is currently listening.
+  bool get isActive => _socket != null && !_disposed;
+
+  /// Starts listening for broadcast messages and enables sending.
+  Future<void> start() async {
+    if (_disposed) return;
+
+    // Acquire Android multicast/broadcast lock.
+    await _acquireMulticastLock();
+
+    try {
+      if (_socketFactory != null) {
+        _socket = await _socketFactory(InternetAddress.anyIPv4, port);
+      } else {
+        _socket = await RawDatagramSocket.bind(
+          InternetAddress.anyIPv4,
+          port,
+          reuseAddress: true,
+        );
+      }
+
+      _socket?.broadcastEnabled = true;
+
+      _socket?.listen(
+        _onSocketEvent,
+        onError: _onError,
+        cancelOnError: false,
+      );
+
+      debugPrint('SyncService: Listening on port $port (device=$deviceId)');
+
+      // Notify other devices that this instance just opened.
+      _sendWake();
+    } on Object catch (e) {
+      debugPrint('SyncService: Failed to start: $e');
+    }
+  }
+
+  /// Broadcasts the given [state] with an [action] label to all peers.
+  void broadcast(PomodoroState state, String action) {
+    if (_socket == null || _disposed) return;
+
+    final message = _encodeMessage(state, action);
+    try {
+      final sent = _socket!.send(
+        message,
+        InternetAddress('255.255.255.255'),
+        port,
+      );
+      debugPrint(
+        'SyncService: Sent $action ($sent bytes) '
+        'to 255.255.255.255:$port',
+      );
+    } on Object catch (e) {
+      debugPrint('SyncService: Send failed: $e');
+    }
+  }
+
+  /// Starts periodic heartbeat that broadcasts current state.
+  ///
+  /// This keeps devices in sync even if an individual message is lost.
+  void startHeartbeat(PomodoroState Function() stateProvider) {
+    _heartbeat?.cancel();
+    _heartbeat = Timer.periodic(
+      const Duration(seconds: 5),
+      (_) => broadcast(stateProvider(), 'heartbeat'),
+    );
+  }
+
+  /// Stops the periodic heartbeat.
+  void stopHeartbeat() {
+    _heartbeat?.cancel();
+    _heartbeat = null;
+  }
+
+  /// Shuts down the sync service.
+  Future<void> dispose() async {
+    _disposed = true;
+    _heartbeat?.cancel();
+    _heartbeat = null;
+
+    _socket?.close();
+    _socket = null;
+
+    await _releaseMulticastLock();
+  }
+
+  // -- Private helpers --
+
+  /// Sends a wake signal to the dedicated wake port so that a desktop
+  /// daemon can auto-launch the app on other devices.
+  void _sendWake() {
+    if (_socket == null || _disposed) return;
+    final message = utf8.encode(jsonEncode(<String, dynamic>{
+      'deviceId': deviceId,
+      'action': 'wake',
+      'timestamp': DateTime.now().millisecondsSinceEpoch,
+    }));
+    try {
+      _socket!.send(message, InternetAddress('255.255.255.255'), wakePort);
+      debugPrint('SyncService: Sent wake to port $wakePort');
+    } on Object catch (e) {
+      debugPrint('SyncService: Wake send failed: $e');
+    }
+  }
+
+  void _onSocketEvent(RawSocketEvent event) {
+    if (event != RawSocketEvent.read) return;
+
+    final datagram = _socket?.receive();
+    if (datagram == null) return;
+
+    try {
+      final json = utf8.decode(datagram.data);
+      final map = jsonDecode(json) as Map<String, dynamic>;
+
+      // Ignore own messages.
+      if (map['deviceId'] == deviceId) return;
+
+      final state = _decodeState(map['state'] as Map<String, dynamic>);
+      final action = map['action'] as String;
+      debugPrint(
+        'SyncService: Received $action from ${map['deviceId']}',
+      );
+      onStateReceived(state, action);
+    } on Object catch (e) {
+      debugPrint('SyncService: Parse error: $e');
+    }
+  }
+
+  void _onError(Object error) {
+    debugPrint('SyncService: Socket error: $error');
+  }
+
+  List<int> _encodeMessage(PomodoroState state, String action) {
+    final map = <String, dynamic>{
+      'deviceId': deviceId,
+      'timestamp': DateTime.now().millisecondsSinceEpoch,
+      'action': action,
+      'state': _encodeState(state),
+    };
+    return utf8.encode(jsonEncode(map));
+  }
+
+  static Map<String, dynamic> _encodeState(PomodoroState state) {
+    return {
+      'mode': state.mode.name,
+      'remainingSeconds': state.remainingSeconds,
+      'totalSeconds': state.totalSeconds,
+      'isRunning': state.isRunning,
+      'completedPomodoros': state.completedPomodoros,
+      'pomodorosPerCycle': state.pomodorosPerCycle,
+    };
+  }
+
+  static PomodoroState _decodeState(Map<String, dynamic> map) {
+    return PomodoroState(
+      mode: PomodoroMode.values.byName(map['mode'] as String),
+      remainingSeconds: map['remainingSeconds'] as int,
+      totalSeconds: map['totalSeconds'] as int,
+      isRunning: map['isRunning'] as bool,
+      completedPomodoros: map['completedPomodoros'] as int,
+      pomodorosPerCycle: map['pomodorosPerCycle'] as int,
+    );
+  }
+
+  static Future<void> _acquireMulticastLock() async {
+    if (!Platform.isAndroid) return;
+    try {
+      await _methodChannel.invokeMethod<bool>('acquire');
+    } on MissingPluginException {
+      // Platform channel not available (e.g., in tests).
+    } on Object catch (e) {
+      debugPrint('SyncService: Failed to acquire multicast lock: $e');
+    }
+  }
+
+  static Future<void> _releaseMulticastLock() async {
+    if (!Platform.isAndroid) return;
+    try {
+      await _methodChannel.invokeMethod<bool>('release');
+    } on MissingPluginException {
+      // Platform channel not available.
+    } on Object catch (e) {
+      debugPrint('SyncService: Failed to release multicast lock: $e');
+    }
+  }
+
+  static String _generateDeviceId() {
+    final random = Random();
+    return List.generate(
+      8,
+      (_) => random.nextInt(256).toRadixString(16).padLeft(2, '0'),
+    ).join();
+  }
+}
diff --git a/pomodoro_app/lib/theme/pomodoro_theme.dart b/pomodoro_app/lib/theme/pomodoro_theme.dart
new file mode 100644
index 0000000..499a138
--- /dev/null
+++ b/pomodoro_app/lib/theme/pomodoro_theme.dart
@@ -0,0 +1,73 @@
+import 'package:flutter/material.dart';
+
+import '../models/pomodoro_state.dart';
+
+/// Provides consistent theming for the Pomodoro app across platforms.
+class PomodoroTheme {
+  PomodoroTheme._();
+
+  // Brand colors per mode.
+  static const Color workColor = Color(0xFFE74C3C);
+  static const Color shortBreakColor = Color(0xFF2ECC71);
+  static const Color longBreakColor = Color(0xFF3498DB);
+
+  static const Color _darkSurface = Color(0xFF1A1A2E);
+  static const Color _darkBackground = Color(0xFF16213E);
+  static const Color _textLight = Color(0xFFF5F5F5);
+  static const Color _textMuted = Color(0xFFB0B0B0);
+
+  /// Returns the accent color for the given [mode].
+  static Color colorForMode(PomodoroMode mode) {
+    switch (mode) {
+      case PomodoroMode.work:
+        return workColor;
+      case PomodoroMode.shortBreak:
+        return shortBreakColor;
+      case PomodoroMode.longBreak:
+        return longBreakColor;
+    }
+  }
+
+  /// The app's dark theme.
+  static ThemeData get darkTheme {
+    return ThemeData(
+      useMaterial3: true,
+      brightness: Brightness.dark,
+      scaffoldBackgroundColor: _darkBackground,
+      colorScheme: const ColorScheme.dark(
+        primary: workColor,
+        surface: _darkSurface,
+        onSurface: _textLight,
+      ),
+      textTheme: const TextTheme(
+        displayLarge: TextStyle(
+          fontSize: 72,
+          fontWeight: FontWeight.w300,
+          color: _textLight,
+          letterSpacing: 4,
+        ),
+        headlineMedium: TextStyle(
+          fontSize: 24,
+          fontWeight: FontWeight.w500,
+          color: _textLight,
+        ),
+        bodyLarge: TextStyle(
+          fontSize: 16,
+          color: _textMuted,
+        ),
+      ),
+      elevatedButtonTheme: ElevatedButtonThemeData(
+        style: ElevatedButton.styleFrom(
+          padding: const EdgeInsets.symmetric(horizontal: 32, vertical: 16),
+          shape: RoundedRectangleBorder(
+            borderRadius: BorderRadius.circular(30),
+          ),
+          textStyle: const TextStyle(
+            fontSize: 16,
+            fontWeight: FontWeight.w600,
+          ),
+        ),
+      ),
+    );
+  }
+}
diff --git a/pomodoro_app/lib/widgets/pomodoro_indicators.dart b/pomodoro_app/lib/widgets/pomodoro_indicators.dart
new file mode 100644
index 0000000..d938cda
--- /dev/null
+++ b/pomodoro_app/lib/widgets/pomodoro_indicators.dart
@@ -0,0 +1,47 @@
+import 'package:flutter/material.dart';
+
+import '../models/pomodoro_state.dart';
+import '../theme/pomodoro_theme.dart';
+
+/// Shows completed pomodoro indicators as filled/unfilled dots.
+class PomodoroIndicators extends StatelessWidget {
+  /// Creates [PomodoroIndicators].
+  const PomodoroIndicators({
+    required this.state,
+    super.key,
+  });
+
+  /// The current Pomodoro state.
+  final PomodoroState state;
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisAlignment: MainAxisAlignment.center,
+      children: List.generate(
+        state.pomodorosPerCycle,
+        (index) {
+          final isCompleted = index < state.completedPomodoros % state.pomodorosPerCycle;
+          return Padding(
+            padding: const EdgeInsets.symmetric(horizontal: 6),
+            child: AnimatedContainer(
+              duration: const Duration(milliseconds: 300),
+              width: 14,
+              height: 14,
+              decoration: BoxDecoration(
+                shape: BoxShape.circle,
+                color: isCompleted
+                    ? PomodoroTheme.workColor
+                    : Colors.white24,
+                border: Border.all(
+                  color: PomodoroTheme.workColor.withValues(alpha: 0.5),
+                  width: 2,
+                ),
+              ),
+            ),
+          );
+        },
+      ),
+    );
+  }
+}
diff --git a/pomodoro_app/lib/widgets/timer_controls.dart b/pomodoro_app/lib/widgets/timer_controls.dart
new file mode 100644
index 0000000..ba369cb
--- /dev/null
+++ b/pomodoro_app/lib/widgets/timer_controls.dart
@@ -0,0 +1,79 @@
+import 'package:flutter/material.dart';
+
+import '../models/pomodoro_state.dart';
+import '../theme/pomodoro_theme.dart';
+
+/// Row of control buttons for the Pomodoro timer.
+class TimerControls extends StatelessWidget {
+  /// Creates [TimerControls].
+  const TimerControls({
+    required this.state,
+    required this.onStart,
+    required this.onPause,
+    required this.onReset,
+    required this.onSkip,
+    super.key,
+  });
+
+  /// The current Pomodoro state.
+  final PomodoroState state;
+
+  /// Callback when user taps start.
+  final VoidCallback onStart;
+
+  /// Callback when user taps pause.
+  final VoidCallback onPause;
+
+  /// Callback when user taps reset.
+  final VoidCallback onReset;
+
+  /// Callback when user taps skip.
+  final VoidCallback onSkip;
+
+  @override
+  Widget build(BuildContext context) {
+    final color = PomodoroTheme.colorForMode(state.mode);
+
+    return Row(
+      mainAxisAlignment: MainAxisAlignment.center,
+      children: [
+        // Reset button.
+        IconButton(
+          onPressed: onReset,
+          icon: const Icon(Icons.refresh),
+          iconSize: 32,
+          tooltip: 'Reset',
+          color: Colors.white70,
+        ),
+        const SizedBox(width: 16),
+        // Play / Pause button.
+        SizedBox(
+          width: 72,
+          height: 72,
+          child: ElevatedButton(
+            onPressed: state.isRunning ? onPause : onStart,
+            style: ElevatedButton.styleFrom(
+              backgroundColor: color,
+              foregroundColor: Colors.white,
+              shape: const CircleBorder(),
+              padding: EdgeInsets.zero,
+            ),
+            child: Icon(
+              state.isRunning ? Icons.pause : Icons.play_arrow,
+              size: 36,
+            ),
+          ),
+        ),
+        const SizedBox(width: 16),
+        // Skip button.
+        IconButton(
+          onPressed: onSkip,
+          icon: const Icon(Icons.skip_next),
+          iconSize: 32,
+          tooltip: 'Skip',
+          color: Colors.white70,
+        ),
+      ],
+    );
+  }
+}
diff --git a/pomodoro_app/lib/widgets/timer_display.dart b/pomodoro_app/lib/widgets/timer_display.dart
new file mode 100644
index 0000000..7b1b86b
--- /dev/null
+++ b/pomodoro_app/lib/widgets/timer_display.dart
@@ -0,0 +1,79 @@
+import 'dart:math';
+
+import 'package:flutter/material.dart';
+
+import '../models/pomodoro_state.dart';
+import '../theme/pomodoro_theme.dart';
+
+/// A circular progress indicator that displays the remaining time.
+class TimerDisplay extends StatelessWidget {
+  /// Creates a [TimerDisplay].
+  const TimerDisplay({
+    required this.state,
+    super.key,
+  });
+
+  /// The current Pomodoro state.
+  final PomodoroState state;
+
+  @override
+  Widget build(BuildContext context) {
+    final color = PomodoroTheme.colorForMode(state.mode);
+
+    return LayoutBuilder(
+      builder: (context, constraints) {
+        final size = min(constraints.maxWidth, constraints.maxHeight) * 0.7;
+        return SizedBox(
+          width: size,
+          height: size,
+          child: Stack(
+            alignment: Alignment.center,
+            children: [
+              // Background circle.
+              SizedBox.expand(
+                child: CircularProgressIndicator(
+                  value: 1.0,
+                  strokeWidth: 8,
+                  color: color.withValues(alpha: 0.2),
+                ),
+              ),
+              // Progress arc.
+              SizedBox.expand(
+                child: CircularProgressIndicator(
+                  value: state.progress,
+                  strokeWidth: 8,
+                  color: color,
+                  strokeCap: StrokeCap.round,
+                ),
+              ),
+              // Time text and mode label.
+              FittedBox(
+                fit: BoxFit.scaleDown,
+                child: Column(
+                  mainAxisSize: MainAxisSize.min,
+                  children: [
+                    Text(
+                      state.modeDisplayLabel,
+                      style: Theme.of(context).textTheme.bodyLarge?.copyWith(
+                            color: color,
+                            fontWeight: FontWeight.w600,
+                          ),
+                    ),
+                    const SizedBox(height: 8),
+                    Text(
+                      state.formattedTime,
+                      style:
+                          Theme.of(context).textTheme.displayLarge?.copyWith(
+                                color: Colors.white,
+                              ),
+                    ),
+                  ],
+                ),
+              ),
+            ],
+          ),
+        );
+      },
+    );
+  }
+}
diff --git a/pomodoro_app/packaging/arch/PKGBUILD b/pomodoro_app/packaging/arch/PKGBUILD
index 80384af..a807ebd 100644
--- a/pomodoro_app/packaging/arch/PKGBUILD
+++ b/pomodoro_app/packaging/arch/PKGBUILD
@@ -47,10 +47,10 @@ package() {
     "$pkgdir/usr/lib/$pkgname/pomodoro_app"
 
   # Install bundled shared libraries.
-  install -Dm644 "$_bundle/lib/libapp.so" \
-    "$pkgdir/usr/lib/$pkgname/lib/libapp.so"
-  install -Dm644 "$_bundle/lib/libflutter_linux_gtk.so" \
-    "$pkgdir/usr/lib/$pkgname/lib/libflutter_linux_gtk.so"
+  for lib in "$_bundle"/lib/*.so; do
+    install -Dm644 "$lib" \
+      "$pkgdir/usr/lib/$pkgname/lib/$(basename "$lib")"
+  done
 
   # Install data directory.
   install -Dm644 "$_bundle/data/icudtl.dat" \
diff --git a/pomodoro_app/test/models/pomodoro_state_test.dart b/pomodoro_app/test/models/pomodoro_state_test.dart
index a9ebd92..46b1750 100644
--- a/pomodoro_app/test/models/pomodoro_state_test.dart
+++ b/pomodoro_app/test/models/pomodoro_state_test.dart
@@ -2,6 +2,27 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:pomodoro_app/models/pomodoro_state.dart';
 
 void main() {
+  group('TimerStyle', () {
+    test('label returns correct strings', () {
+      expect(TimerStyle.pomodoro.label, 'Pomodoro');
+      expect(TimerStyle.ultraradian.label, 'Ultraradian');
+    });
+
+    test('pomodoro has correct defaults', () {
+      expect(TimerStyle.pomodoro.defaultWorkMinutes, 25);
+      expect(TimerStyle.pomodoro.defaultShortBreakMinutes, 5);
+      expect(TimerStyle.pomodoro.defaultLongBreakMinutes, 15);
+      expect(TimerStyle.pomodoro.defaultPomodorosPerCycle, 4);
+    });
+
+    test('ultraradian has correct defaults', () {
+      expect(TimerStyle.ultraradian.defaultWorkMinutes, 90);
+      expect(TimerStyle.ultraradian.defaultShortBreakMinutes, 30);
+      expect(TimerStyle.ultraradian.defaultLongBreakMinutes, 30);
+      expect(TimerStyle.ultraradian.defaultPomodorosPerCycle, 1);
+    });
+  });
+
   group('PomodoroMode', () {
     test('label returns correct strings', () {
       expect(PomodoroMode.work.label, 'Work');
@@ -98,6 +119,30 @@ void main() {
     });
   });
 
+  group('PomodoroState.modeDisplayLabel', () {
+    test('returns mode label when pomodorosPerCycle > 1', () {
+      final state = PomodoroState.initial();
+      expect(state.modeDisplayLabel, 'Work');
+
+      final breakState = state.copyWith(mode: PomodoroMode.shortBreak);
+      expect(breakState.modeDisplayLabel, 'Short Break');
+
+      final longBreakState = state.copyWith(mode: PomodoroMode.longBreak);
+      expect(longBreakState.modeDisplayLabel, 'Long Break');
+    });
+
+    test('returns Break when pomodorosPerCycle is 1 and not work', () {
+      final state = PomodoroState.initial(pomodorosPerCycle: 1);
+      expect(state.modeDisplayLabel, 'Work');
+
+      final breakState = state.copyWith(mode: PomodoroMode.shortBreak);
+      expect(breakState.modeDisplayLabel, 'Break');
+
+      final longBreakState = state.copyWith(mode: PomodoroMode.longBreak);
+      expect(longBreakState.modeDisplayLabel, 'Break');
+    });
+  });
+
   group('PomodoroState equality', () {
     test('equal states are ==', () {
       final a = PomodoroState.initial();
diff --git a/pomodoro_app/test/screens/pomodoro_screen_test.dart b/pomodoro_app/test/screens/pomodoro_screen_test.dart
index d533b51..c460db4 100644
--- a/pomodoro_app/test/screens/pomodoro_screen_test.dart
+++ b/pomodoro_app/test/screens/pomodoro_screen_test.dart
@@ -174,5 +174,34 @@ void main() {
       // We can check that the PomodoroIndicators widget is present.
       expect(find.text('0 pomodoros completed'), findsOneWidget);
     });
+
+    testWidgets('shows style picker with Pomodoro selected', (tester) async {
+      await tester.pumpWidget(createApp());
+      expect(find.text('Pomodoro'), findsOneWidget);
+      expect(find.text('Ultraradian'), findsOneWidget);
+    });
+
+    testWidgets('switching to ultraradian updates timer', (tester) async {
+      await tester.pumpWidget(createApp());
+
+      await tester.tap(find.text('Ultraradian'));
+      await tester.pump();
+
+      // Ultraradian work session is 90 minutes.
+      expect(find.text('90:00'), findsOneWidget);
+      expect(timer.timerStyle, TimerStyle.ultraradian);
+    });
+
+    testWidgets('switching back to pomodoro resets timer', (tester) async {
+      await tester.pumpWidget(createApp());
+
+      await tester.tap(find.text('Ultraradian'));
+      await tester.pump();
+      expect(find.text('90:00'), findsOneWidget);
+
+      await tester.tap(find.text('Pomodoro'));
+      await tester.pump();
+      expect(find.text('25:00'), findsOneWidget);
+    });
   });
 }
diff --git a/pomodoro_app/test/services/notification_service_test.dart b/pomodoro_app/test/services/notification_service_test.dart
new file mode 100644
index 0000000..908c525
--- /dev/null
+++ b/pomodoro_app/test/services/notification_service_test.dart
@@ -0,0 +1,211 @@
+import 'dart:io';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:pomodoro_app/models/pomodoro_state.dart';
+import 'package:pomodoro_app/services/notification_service.dart';
+
+/// Captured call to the mock process runner.
+class _Call {
+  _Call(this.executable, this.args);
+  final String executable;
+  final List<String> args;
+}
+
+void main() {
+  group('NotificationService', () {
+    late List<_Call> calls;
+    late NotificationService service;
+
+    Future<ProcessResult> mockRun(String exec, List<String> args) async {
+      calls.add(_Call(exec, args));
+      return ProcessResult(0, 0, '(uint32 42,)', '');
+    }
+
+    setUp(() {
+      calls = [];
+      service = NotificationService(runProcess: mockRun);
+    });
+
+    tearDown(() {
+      service.dispose();
+    });
+
+    test('showTimer sends Notify via gdbus', () async {
+      final state = PomodoroState(
+        mode: PomodoroMode.work,
+        remainingSeconds: 1500,
+        totalSeconds: 1500,
+        isRunning: true,
+        completedPomodoros: 0,
+        pomodorosPerCycle: 4,
+      );
+
+      await service.showTimer(state: state);
+
+      expect(calls, hasLength(1));
+      expect(calls[0].executable, 'gdbus');
+      expect(
+        calls[0].args,
+        contains('org.freedesktop.Notifications.Notify'),
+      );
+      expect(calls[0].args, contains('Work \u2013 25:00'));
+      expect(calls[0].args, contains("['pause', 'Pause', 'skip', 'Skip']"));
+    });
+
+    test('showTimer shows Start action when paused', () async {
+      final state = PomodoroState(
+        mode: PomodoroMode.shortBreak,
+        remainingSeconds: 120,
+        totalSeconds: 300,
+        isRunning: false,
+        completedPomodoros: 1,
+        pomodorosPerCycle: 4,
+      );
+
+      await service.showTimer(state: state);
+
+      expect(calls[0].args, contains("['start', 'Start']"));
+    });
+
+    test('showTimer replaces previous notification', () async {
+      final state = PomodoroState(
+        mode: PomodoroMode.work,
+        remainingSeconds: 1500,
+        totalSeconds: 1500,
+        isRunning: true,
+        completedPomodoros: 0,
+        pomodorosPerCycle: 4,
+      );
+
+      await service.showTimer(state: state);
+
+      // First call should use replaces_id 0.
+      expect(calls[0].args, contains('0'));
+
+      // Second call should use the parsed ID 42.
+      await service.showTimer(state: state);
+      expect(calls[1].args, contains('42'));
+    });
+
+    test('parses notification ID from gdbus output', () async {
+      final state = PomodoroState.initial();
+
+      await service.showTimer(state: state);
+      expect(service.currentId, 42);
+    });
+
+    test('handles unparsable gdbus output gracefully', () async {
+      final stubService = NotificationService(
+        runProcess: (exec, args) async {
+          return ProcessResult(0, 0, 'unexpected output', '');
+        },
+      );
+
+      final state = PomodoroState.initial();
+      await stubService.showTimer(state: state);
+      expect(stubService.currentId, 0);
+
+      stubService.dispose();
+    });
+
+    test('showSessionComplete sends correct content', () async {
+      await service.showSessionComplete(
+        completedMode: PomodoroMode.work,
+        nextMode: PomodoroMode.shortBreak,
+      );
+
+      expect(calls, hasLength(1));
+      expect(calls[0].args, contains('Work complete!'));
+      expect(calls[0].args, contains('Up next: Short Break'));
+    });
+
+    test('cancel sends CloseNotification', () async {
+      // First show a notification to get an ID.
+      final state = PomodoroState.initial();
+      await service.showTimer(state: state);
+      calls.clear();
+
+      await service.cancel();
+
+      expect(calls, hasLength(1));
+      expect(
+        calls[0].args,
+        contains('org.freedesktop.Notifications.CloseNotification'),
+      );
+      expect(calls[0].args, contains('42'));
+    });
+
+    test('cancel does nothing when no notification shown', () async {
+      await service.cancel();
+      expect(calls, isEmpty);
+    });
+
+    test('cancel resets currentId to 0', () async {
+      final state = PomodoroState.initial();
+      await service.showTimer(state: state);
+      expect(service.currentId, 42);
+
+      await service.cancel();
+      expect(service.currentId, 0);
+    });
+
+    test('does nothing after dispose', () async {
+      service.dispose();
+
+      final state = PomodoroState.initial();
+      await service.showTimer(state: state);
+      await service.showSessionComplete(
+        completedMode: PomodoroMode.work,
+        nextMode: PomodoroMode.shortBreak,
+      );
+      await service.cancel();
+
+      expect(calls, isEmpty);
+    });
+
+    test('dispose cancels active notification', () async {
+      final state = PomodoroState.initial();
+      await service.showTimer(state: state);
+      calls.clear();
+
+      service.dispose();
+
+      // Cancel was fired (fire-and-forget).
+      expect(calls, hasLength(1));
+      expect(
+        calls[0].args,
+        contains('org.freedesktop.Notifications.CloseNotification'),
+      );
+    });
+
+    test('handles process error gracefully', () async {
+      final errorService = NotificationService(
+        runProcess: (exec, args) async {
+          throw const OSError('gdbus not found');
+        },
+      );
+
+      final state = PomodoroState.initial();
+      // Should not throw.
+      await errorService.showTimer(state: state);
+      await errorService.cancel();
+
+      errorService.dispose();
+    });
+  });
+
+  group('progressBar', () {
+    test('returns empty bar at 0%', () {
+      expect(NotificationService.progressBar(0.0), '░' * 20);
+    });
+
+    test('returns full bar at 100%', () {
+      expect(NotificationService.progressBar(1.0), '█' * 20);
+    });
+
+    test('returns half bar at 50%', () {
+      final bar = NotificationService.progressBar(0.5);
+      expect(bar, '${'█' * 10}${'░' * 10}');
+    });
+  });
+}
diff --git a/pomodoro_app/test/services/pomodoro_timer_test.dart b/pomodoro_app/test/services/pomodoro_timer_test.dart
index e071bca..b26ef26 100644
--- a/pomodoro_app/test/services/pomodoro_timer_test.dart
+++ b/pomodoro_app/test/services/pomodoro_timer_test.dart
@@ -247,6 +247,70 @@ void main() {
     });
   });
 
+  group('switchStyle()', () {
+    test('switches to ultraradian with correct durations', () {
+      timer.switchStyle(TimerStyle.ultraradian);
+      expect(timer.timerStyle, TimerStyle.ultraradian);
+      expect(timer.state.remainingSeconds, 90 * 60);
+      expect(timer.state.totalSeconds, 90 * 60);
+      expect(timer.state.pomodorosPerCycle, 1);
+      expect(timer.state.mode, PomodoroMode.work);
+      expect(timer.state.isRunning, false);
+    });
+
+    test('switches back to pomodoro', () {
+      timer.switchStyle(TimerStyle.ultraradian);
+      timer.switchStyle(TimerStyle.pomodoro);
+      expect(timer.timerStyle, TimerStyle.pomodoro);
+      expect(timer.state.remainingSeconds, 25 * 60);
+      expect(timer.state.totalSeconds, 25 * 60);
+      expect(timer.state.pomodorosPerCycle, 4);
+    });
+
+    test('resets running timer when switching', () {
+      timer.start();
+      fakeController.tick();
+      expect(timer.state.isRunning, true);
+
+      timer.switchStyle(TimerStyle.ultraradian);
+      expect(timer.state.isRunning, false);
+      expect(timer.state.remainingSeconds, 90 * 60);
+    });
+
+    test('does nothing when switching to same style', () {
+      timer.start();
+      fakeController.tick();
+      final stateBefore = timer.state;
+
+      timer.switchStyle(TimerStyle.pomodoro);
+      expect(timer.state, stateBefore);
+    });
+
+    test('notifies listeners', () {
+      var notified = false;
+      timer.addListener(() => notified = true);
+      timer.switchStyle(TimerStyle.ultraradian);
+      expect(notified, true);
+    });
+
+    test('resets completed pomodoros', () {
+      timer.start();
+      for (var i = 0; i < 60; i++) {
+        fakeController.tick();
+      }
+      expect(timer.state.completedPomodoros, 1);
+
+      timer.switchStyle(TimerStyle.ultraradian);
+      expect(timer.state.completedPomodoros, 0);
+    });
+  });
+
+  group('timerStyle getter', () {
+    test('defaults to pomodoro', () {
+      expect(timer.timerStyle, TimerStyle.pomodoro);
+    });
+  });
+
   group('applyRemoteState()', () {
     test('applies remote state and notifies listeners', () {
       var notified = false;
diff --git a/pomodoro_app/test/services/sync_service_test.dart b/pomodoro_app/test/services/sync_service_test.dart
index 05ed281..69befd3 100644
--- a/pomodoro_app/test/services/sync_service_test.dart
+++ b/pomodoro_app/test/services/sync_service_test.dart
@@ -10,12 +10,12 @@ import 'package:pomodoro_app/services/sync_service.dart';
 /// injecting received messages.
 class FakeDatagramSocket implements RawDatagramSocket {
   final _controller = StreamController<RawSocketEvent>.broadcast();
-  final List<_SentDatagram> sentMessages = [];
+  final List<SentDatagram> sentMessages = [];
   Datagram? _pendingDatagram;
 
   @override
   int send(List<int> buffer, InternetAddress address, int port) {
-    sentMessages.add(_SentDatagram(buffer, address, port));
+    sentMessages.add(SentDatagram(buffer, address, port));
     return buffer.length;
   }
 
@@ -61,8 +61,8 @@ class FakeDatagramSocket implements RawDatagramSocket {
   dynamic noSuchMethod(Invocation invocation) => null;
 }
 
-class _SentDatagram {
-  _SentDatagram(this.data, this.address, this.port);
+class SentDatagram {
+  SentDatagram(this.data, this.address, this.port);
   final List<int> data;
   final InternetAddress address;
   final int port;
@@ -118,7 +118,6 @@ void main() {
     });
 
     test('ignores own messages', () async {
-      final state = PomodoroState.initial();
       final message = jsonEncode({
         'deviceId': 'test-device-1', // Same as our device.
         'timestamp': DateTime.now().millisecondsSinceEpoch,
@@ -212,7 +211,7 @@ void main() {
         PomodoroState? received;
 
         final sender = SyncService(
-          onStateReceived: (_, __) {},
+          onStateReceived: (_, _) {},
           deviceId: 'sender',
           socketFactory: (h, p) async => fakeSocket,
         );