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/android_scan.dart

@@ -34,11 +34,11 @@ Future<int> scanAndroidMediaStore(
 
   final now = DateTime.now().millisecondsSinceEpoch;
   final companions = <SongsCompanion>[];
-  final livePaths = <String>[];
   var done = 0;
 
+  if (songs.isNotEmpty) onProgress?.call(0, songs.length);
+
   for (final s in songs) {
-    livePaths.add(s.data);
     final prev = existing[s.data];
     final id = prev?.id ?? _uuid.v4();
 
@@ -76,7 +76,7 @@ Future<int> scanAndroidMediaStore(
   }
 
   await db.upsertSongs(companions);
-  if (livePaths.isNotEmpty) {
+  if (companions.isNotEmpty) {
     await db.markMissing(now);
   }
   return companions.length;

lib/library/library_screen.dart

@@ -49,7 +49,7 @@ class LibraryScreen extends StatelessWidget {
               content: Text(lib.scanError!),
               actions: [
                 TextButton(
-                  onPressed: () => lib.scanError = null,
+                  onPressed: lib.dismissScanError,
                   child: const Text('Verwerfen'),
                 ),
               ],

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;

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;

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')),
+                );
+              }
+            }
+          },
         );
       },
     );