Fix critical blocker issues and improve tests

## Critical Blockers Fixed
- Remove playbackState.addError() which broke audio_service permanently
  - Move error handling from audio_handler to song_list where UI context exists
  - Display error in SnackBar instead of destroying playback stream

- Add escapeChar parameter to LIKE queries for correct wildcard handling
  - Fixes broken search for titles with _ or % characters

- Add dismissScanError() method to allow closing error banner
  - Previously banner couldn't be dismissed (missing notifyListeners())

## Improvements
- Remove dead livePaths list, use companions.isNotEmpty instead
- Add initial progress update to show total files when scan starts
- Add tooltip to now_playing_screen Zurück button
- Add proper error handling in song_list with async/await

## Testing
- Add regression test for CRITICAL-1: empty scan doesn't delete songs
- Add regression test for CRITICAL-2: tombstoned files restore on re-scan
- All 11 tests pass

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/library/library_service.dart

@@ -57,6 +57,11 @@ class LibraryService extends ChangeNotifier {
     return granted;
   }
 
+  void dismissScanError() {
+    scanError = null;
+    notifyListeners();
+  }
+
   Future<void> _scan() async {
     if (scanning) return;
     scanning = true;