feat(playlists): add Playlisten-Tab mit Favoriten-Kurzeinstieg und Erstellen-Dialog

- PlaylistsScreen: Favoriten-ListTile oben (watchFavorites, pusht FavoritesListScreen),
  darunter alle Playlisten (watchPlaylists) mit Songanzahl, Löschen via Icon/Long-Press
  + Bestätigungsdialog
- CreatePlaylistDialog: Name-Eingabe, Erstellen-Button erst aktiv bei nicht-leerem Text,
  gibt die neue Playlist-ID zurück
- Leer-Zustand "Noch keine Playlisten" nach Vorbild von library_screen.dart
- main.dart: Playlisten-Platzhalter durch PlaylistsScreen ersetzt

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

test/playlists/playlists_screen_test.dart

@@ -0,0 +1,45 @@
+import 'package:drift/native.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:provider/provider.dart';
+import 'package:melo/library/database.dart';
+import 'package:melo/library/playlist_service.dart';
+import 'package:melo/playlists/playlists_screen.dart';
+
+Widget _wrap(MeloDb db, PlaylistService service, Widget child) {
+  return MultiProvider(
+    providers: [
+      Provider<MeloDb>.value(value: db),
+      ChangeNotifierProvider<PlaylistService>.value(value: service),
+    ],
+    child: MaterialApp(home: child),
+  );
+}
+
+void main() {
+  testWidgets('zeigt Leer-Zustand ohne Playlisten', (tester) async {
+    final db = MeloDb(NativeDatabase.memory());
+    final service = PlaylistService(db);
+
+    await tester.pumpWidget(_wrap(db, service, const PlaylistsScreen()));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Noch keine Playlisten'), findsOneWidget);
+
+    await db.close();
+  });
+
+  testWidgets('zeigt angelegte Playlist in der Liste', (tester) async {
+    final db = MeloDb(NativeDatabase.memory());
+    final service = PlaylistService(db);
+    await db.createPlaylist('Road Trip');
+
+    await tester.pumpWidget(_wrap(db, service, const PlaylistsScreen()));
+    await tester.pumpAndSettle();
+
+    expect(find.text('Road Trip'), findsOneWidget);
+    expect(find.text('0 Songs'), findsWidgets);
+
+    await db.close();
+  });
+}