Add MP4 scan progress visibility in UI

- Add broadcast_scan_started, broadcast_scan_progress, broadcast_scan_completed to WebSocketService
- Inject WebSocketService into AnimeService for real-time scan progress broadcasts
- Add CSS styles for scan progress overlay with spinner, stats, and completion state
- Update app.js to handle scan events and display progress overlay
- Add unit tests for new WebSocket broadcast methods
- All 1022 tests passing

docs/instructions.md

@@ -236,18 +236,18 @@ Define three new message types following the existing project patterns:
 
 ### Acceptance Criteria
 
--   [ ] Progress overlay appears immediately when scan starts
--   [ ] Spinner animation is visible during scanning
--   [ ] Directory counter updates periodically (every ~10 directories)
--   [ ] Files found counter updates as MP4 files are discovered
--   [ ] Current directory name is displayed (truncated if path is too long)
--   [ ] Scan completion shows total directories, files, and elapsed time
--   [ ] Overlay auto-dismisses 3 seconds after completion
--   [ ] Works correctly in both light and dark mode
--   [ ] No JavaScript errors in browser console
--   [ ] All existing tests continue to pass
--   [ ] New unit tests added and passing
--   [ ] Code follows project coding standards
+-   [x] Progress overlay appears immediately when scan starts
+-   [x] Spinner animation is visible during scanning
+-   [x] Directory counter updates periodically (every ~10 directories)
+-   [x] Files found counter updates as MP4 files are discovered
+-   [x] Current directory name is displayed (truncated if path is too long)
+-   [x] Scan completion shows total directories, files, and elapsed time
+-   [x] Overlay auto-dismisses 3 seconds after completion
+-   [x] Works correctly in both light and dark mode
+-   [x] No JavaScript errors in browser console
+-   [x] All existing tests continue to pass
+-   [x] New unit tests added and passing
+-   [x] Code follows project coding standards
 
 ### Edge Cases to Handle