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/song_list.dart

@@ -27,7 +27,17 @@ class SongList extends StatelessWidget {
           title: Text(s.title, maxLines: 1, overflow: TextOverflow.ellipsis),
           subtitle: Text(s.artist ?? 'Unbekannt',
               maxLines: 1, overflow: TextOverflow.ellipsis),
-          onTap: () => playSongs(handler, songs, i),
+          onTap: () async {
+            try {
+              await playSongs(handler, songs, i);
+            } catch (e) {
+              if (context.mounted) {
+                ScaffoldMessenger.of(context).showSnackBar(
+                  SnackBar(content: Text('Wiedergabe fehlgeschlagen: $e')),
+                );
+              }
+            }
+          },
         );
       },
     );