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>

lib/main.dart

@@ -10,6 +10,7 @@ import 'library/search_screen.dart';
 import 'player/audio_handler.dart';
 import 'player/mini_player.dart';
 import 'player/now_playing_screen.dart';
+import 'playlists/playlists_screen.dart';
 import 'shared/theme.dart';
 
 late final MeloAudioHandler _handler;
@@ -68,7 +69,7 @@ class _HomeShellState extends State<HomeShell> {
   static const _tabs = <Widget>[
     NowPlayingScreen(),
     LibraryScreen(),
-    _Placeholder(label: 'Playlisten', hint: 'Kommt in Kürze'),
+    PlaylistsScreen(),
     SearchScreen(),
     _Placeholder(label: 'Settings', hint: 'Kommt in Kürze'),
   ];

lib/playlists/create_playlist_dialog.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../library/playlist_service.dart';
+
+/// Dialog zum Anlegen einer neuen Playlist. Gibt bei Erfolg die neue
+/// Playlist-ID zurück (z.B. um direkt danach einen Song hinzuzufügen).
+class CreatePlaylistDialog extends StatefulWidget {
+  const CreatePlaylistDialog({super.key});
+
+  @override
+  State<CreatePlaylistDialog> createState() => _CreatePlaylistDialogState();
+}
+
+class _CreatePlaylistDialogState extends State<CreatePlaylistDialog> {
+  final _controller = TextEditingController();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  Future<void> _create() async {
+    final name = _controller.text.trim();
+    if (name.isEmpty) return;
+    final id = await context.read<PlaylistService>().createPlaylist(name);
+    if (mounted) Navigator.pop(context, id);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final canCreate = _controller.text.trim().isNotEmpty;
+    return AlertDialog(
+      title: const Text('Neue Playlist'),
+      content: TextField(
+        controller: _controller,
+        autofocus: true,
+        decoration: const InputDecoration(hintText: 'Name der Playlist'),
+        onChanged: (_) => setState(() {}),
+        onSubmitted: (_) => canCreate ? _create() : null,
+      ),
+      actions: [
+        TextButton(
+          onPressed: () => Navigator.pop(context),
+          child: const Text('Abbrechen'),
+        ),
+        TextButton(
+          onPressed: canCreate ? _create : null,
+          child: const Text('Erstellen'),
+        ),
+      ],
+    );
+  }
+}

lib/playlists/playlists_screen.dart

@@ -0,0 +1,171 @@
+import 'package:flutter/material.dart';
+import 'package:provider/provider.dart';
+
+import '../library/database.dart';
+import '../library/playlist_service.dart';
+import '../library/song_list.dart';
+import '../shared/theme.dart';
+import 'create_playlist_dialog.dart';
+import 'playlist_detail_screen.dart';
+
+/// Playlisten-Tab: Favoriten-Kurzeinstieg oben, darunter alle eigenen Playlisten.
+class PlaylistsScreen extends StatelessWidget {
+  const PlaylistsScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final db = context.read<MeloDb>();
+    return Scaffold(
+      appBar: AppBar(title: const Text('Playlisten')),
+      body: Column(
+        children: [
+          StreamBuilder<List<Song>>(
+            stream: db.watchFavorites(),
+            builder: (context, snapshot) {
+              final count = snapshot.data?.length ?? 0;
+              return ListTile(
+                leading: const Icon(Icons.favorite, color: MeloTheme.red),
+                title: const Text('Favoriten'),
+                subtitle: Text('$count Songs'),
+                onTap: () => Navigator.push(
+                  context,
+                  MaterialPageRoute(builder: (_) => const FavoritesListScreen()),
+                ),
+              );
+            },
+          ),
+          const Divider(height: 1),
+          Expanded(
+            child: StreamBuilder<List<Playlist>>(
+              stream: db.watchPlaylists(),
+              builder: (context, snapshot) {
+                if (snapshot.hasError) {
+                  return Center(
+                    child: Text('Fehler: ${snapshot.error}',
+                        style: const TextStyle(color: Colors.white54)),
+                  );
+                }
+                final playlists = snapshot.data ?? const [];
+                if (playlists.isEmpty) {
+                  return const _Empty();
+                }
+                return ListView.builder(
+                  itemCount: playlists.length,
+                  itemBuilder: (context, i) => _PlaylistTile(playlist: playlists[i]),
+                );
+              },
+            ),
+          ),
+        ],
+      ),
+      floatingActionButton: FloatingActionButton(
+        tooltip: 'Neue Playlist',
+        onPressed: () => showDialog<String?>(
+          context: context,
+          builder: (_) => const CreatePlaylistDialog(),
+        ),
+        child: const Icon(Icons.add),
+      ),
+    );
+  }
+}
+
+class _PlaylistTile extends StatelessWidget {
+  const _PlaylistTile({required this.playlist});
+  final Playlist playlist;
+
+  Future<void> _confirmDelete(BuildContext context) async {
+    final confirmed = await showDialog<bool>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: const Text('Playlist löschen?'),
+        content: Text('„${playlist.name}“ wird dauerhaft gelöscht.'),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, false),
+            child: const Text('Abbrechen'),
+          ),
+          TextButton(
+            onPressed: () => Navigator.pop(ctx, true),
+            child: const Text('Löschen'),
+          ),
+        ],
+      ),
+    );
+    if (confirmed == true && context.mounted) {
+      await context.read<PlaylistService>().deletePlaylist(playlist.id);
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final db = context.read<MeloDb>();
+    return StreamBuilder<List<Song>>(
+      stream: db.watchPlaylistSongs(playlist.id),
+      builder: (context, snapshot) {
+        final count = snapshot.data?.length ?? 0;
+        return ListTile(
+          leading: const Icon(Icons.playlist_play),
+          title: Text(playlist.name),
+          subtitle: Text('$count Songs'),
+          trailing: IconButton(
+            tooltip: 'Playlist löschen',
+            icon: const Icon(Icons.delete_outline),
+            onPressed: () => _confirmDelete(context),
+          ),
+          onLongPress: () => _confirmDelete(context),
+          onTap: () => Navigator.push(
+            context,
+            MaterialPageRoute(builder: (_) => PlaylistDetailScreen(playlist: playlist)),
+          ),
+        );
+      },
+    );
+  }
+}
+
+class _Empty extends StatelessWidget {
+  const _Empty();
+
+  @override
+  Widget build(BuildContext context) {
+    return const Center(
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        children: [
+          Icon(Icons.playlist_play, size: 64, color: Colors.white24),
+          SizedBox(height: 12),
+          Text('Noch keine Playlisten', style: TextStyle(color: Colors.white54)),
+          SizedBox(height: 8),
+          Text('Tippe auf + um eine Playlist zu erstellen',
+              style: TextStyle(color: Colors.white38, fontSize: 12)),
+        ],
+      ),
+    );
+  }
+}
+
+/// Gepushte Ansicht der favorisierten Songs.
+class FavoritesListScreen extends StatelessWidget {
+  const FavoritesListScreen({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    final db = context.read<MeloDb>();
+    return Scaffold(
+      appBar: AppBar(title: const Text('Favoriten')),
+      body: StreamBuilder<List<Song>>(
+        stream: db.watchFavorites(),
+        builder: (context, snapshot) {
+          final songs = snapshot.data ?? const [];
+          if (songs.isEmpty) {
+            return const Center(
+              child: Text('Noch keine Favoriten', style: TextStyle(color: Colors.white54)),
+            );
+          }
+          return SongList(songs);
+        },
+      ),
+    );
+  }
+}