Add scripts/check_file_length.py and a max-file-length pre-commit hook that
fails any Python/shell file exceeding 400 lines. Extract UIWidgetsMixin and
UIFlowsRelaxedMixin from screen_lock.py and _ui_flows.py respectively, and
split 6 oversized test files into part2/part3/part4 siblings.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>