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/shared/favorite_button.dart

@@ -0,0 +1,31 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+import '../library/database.dart';
+import '../library/playlist_service.dart';
+import 'theme.dart';
+
+/// Herz-Toggle: ausgefüllt = favorisiert, umrandet = nicht favorisiert.
+class FavoriteButton extends StatelessWidget {
+  const FavoriteButton({super.key, required this.songId});
+  final String songId;
+
+  @override
+  Widget build(BuildContext context) {
+    final db = context.read<MeloDb>();
+    final service = context.read<PlaylistService>();
+    return StreamBuilder<bool>(
+      stream: db.watchIsFavorite(songId),
+      builder: (context, snapshot) {
+        final isFavorite = snapshot.data ?? false;
+        return IconButton(
+          tooltip: isFavorite ? 'Favorit entfernen' : 'Zu Favoriten hinzufügen',
+          icon: Icon(
+            isFavorite ? Icons.favorite : Icons.favorite_border,
+            color: isFavorite ? MeloTheme.red : Colors.white54,
+          ),
+          onPressed: () => service.toggleFavorite(songId),
+        );
+      },
+    );
+  }
+}