feat(ui): add favorite heart toggle, wire into song list

- Filled heart when favorited, outline when not (reactive via watchIsFavorite)
- Wired into every song list tile (library, search, playlists)

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

lib/library/song_list.dart

@@ -3,6 +3,7 @@ import 'package:provider/provider.dart';
 
 import '../player/audio_handler.dart';
 import '../shared/cover.dart';
+import '../shared/favorite_button.dart';
 import 'database.dart';
 import 'song_media.dart';
 
@@ -27,6 +28,7 @@ class SongList extends StatelessWidget {
           title: Text(s.title, maxLines: 1, overflow: TextOverflow.ellipsis),
           subtitle: Text(s.artist ?? 'Unbekannt',
               maxLines: 1, overflow: TextOverflow.ellipsis),
+          trailing: FavoriteButton(songId: s.id),
           onTap: () async {
             try {
               await playSongs(handler, songs, i);