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/player/audio_handler.dart

@@ -34,12 +34,8 @@ class MeloAudioHandler extends BaseAudioHandler with QueueHandler, SeekHandler {
     final sources = items
         .map((item) => AudioSource.uri(Uri.parse(item.id), tag: item))
         .toList();
-    try {
-      await _player.setAudioSources(sources, initialIndex: startIndex);
-      await play();
-    } catch (e) {
-      playbackState.addError(e);
-    }
+    await _player.setAudioSources(sources, initialIndex: startIndex);
+    await play();
   }
 
   @override

lib/player/now_playing_screen.dart

@@ -17,6 +17,7 @@ class NowPlayingScreen extends StatelessWidget {
       appBar: AppBar(
         backgroundColor: Colors.transparent,
         leading: IconButton(
+          tooltip: 'Schließen',
           icon: const Icon(Icons.keyboard_arrow_down),
           onPressed: () => Navigator.of(context).maybePop(),
         ),