🌟 Feature: Favoriten-Sync vom Navidrome-Server (Phase 2, Part 4/X)

- syncFavoritesFromServer() in PlaylistService laden Server-Favoriten
- Prüft lokal existierende Songs und markiert als Favorit
- Settings-Button: "🌟 Favoriten-Sync" mit Cloud-Download Icon
- Zeigt Anzahl importierter Favoriten mit SnackBar
- DB-Methoden hinzugefügt: songExists() + setFavorite()

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XJzQjUtnvYUtHdnTs3iCru

lib/library/database.dart

@@ -247,6 +247,23 @@ class MeloDb extends _$MeloDb {
   }
 
   // === Favorites ===
+  Future<bool> songExists(String songId) async {
+    final row = await (select(songs)..where((s) => s.id.equals(songId))).getSingleOrNull();
+    return row != null;
+  }
+
+  Future<void> setFavorite(String songId, bool isFavorite) async {
+    final existing = await (select(favorites)..where((f) => f.songId.equals(songId))).getSingleOrNull();
+    if (isFavorite && existing == null) {
+      await into(favorites).insert(FavoritesCompanion.insert(
+        songId: songId,
+        createdAtMs: DateTime.now().millisecondsSinceEpoch,
+      ));
+    } else if (!isFavorite && existing != null) {
+      await (delete(favorites)..where((f) => f.songId.equals(songId))).go();
+    }
+  }
+
   Future<void> toggleFavorite(String songId) async {
     final existing = await (select(favorites)..where((f) => f.songId.equals(songId))).getSingleOrNull();
     if (existing == null) {