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/scan_service.dart

@@ -45,11 +45,11 @@ Future<int> scanFolders(
 
   final now = DateTime.now().millisecondsSinceEpoch;
   final companions = <SongsCompanion>[];
-  final livePaths = <String>[];
   var done = 0;
 
+  if (files.isNotEmpty) onProgress?.call(0, files.length);
+
   for (final file in files) {
-    livePaths.add(file.path);
     final prev = existing[file.path];
     final id = prev?.id ?? _uuid.v4();
 
@@ -96,7 +96,7 @@ Future<int> scanFolders(
   }
 
   await db.upsertSongs(companions);
-  if (livePaths.isNotEmpty) {
+  if (companions.isNotEmpty) {
     await db.markMissing(now);
   }
   return companions.length;