feat(player): Now Playing als Tab statt Push-Screen, Favoriten-Herz

- songToMediaItem trägt jetzt die Song-UUID in extras['songId'], damit
  Downstream-Code (Favoriten-Button, später Playback-History) den echten
  Song wiederfinden kann statt nur die file-URI zu haben
- NowPlayingScreen: Schließen-Button entfernt (AppBar-leading), da der
  Screen jetzt als eigener Tab läuft statt gepusht zu werden
- FavoriteButton für den aktuellen Song in die AppBar-Actions eingebaut,
  reaktiv über einen verschachtelten StreamBuilder<MediaItem?>; ohne
  gültige songId (z.B. nichts spielt) wird kein Button gerendert

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

lib/library/song_media.dart

@@ -10,6 +10,7 @@ MediaItem songToMediaItem(Song s) => MediaItem(
       album: s.album,
       duration: s.durationMs != null ? Duration(milliseconds: s.durationMs!) : null,
       artUri: s.coverPath != null ? Uri.file(s.coverPath!) : null,
+      extras: {'songId': s.id},
     );
 
 /// Spielt [songs] ab [startIndex] ab.

lib/player/now_playing_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 import '../shared/cover.dart';
+import '../shared/favorite_button.dart';
 import '../shared/theme.dart';
 import 'audio_handler.dart';
 
@@ -16,12 +17,15 @@ class NowPlayingScreen extends StatelessWidget {
     return Scaffold(
       appBar: AppBar(
         backgroundColor: Colors.transparent,
-        leading: IconButton(
-          tooltip: 'Schließen',
-          icon: const Icon(Icons.keyboard_arrow_down),
-          onPressed: () => Navigator.of(context).maybePop(),
-        ),
         actions: [
+          StreamBuilder<MediaItem?>(
+            stream: handler.mediaItem,
+            builder: (context, snapshot) {
+              final songId = snapshot.data?.extras?['songId'] as String?;
+              if (songId == null) return const SizedBox.shrink();
+              return FavoriteButton(songId: songId);
+            },
+          ),
           _SleepTimerButton(handler: handler),
         ],
       ),

test/library/song_media_test.dart

@@ -0,0 +1,24 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:melo/library/database.dart';
+import 'package:melo/library/song_media.dart';
+
+void main() {
+  test('songToMediaItem trägt die Song-UUID in extras', () {
+    final song = Song(
+      id: 'song-1',
+      path: '/a.mp3',
+      title: 'A',
+      artist: null,
+      album: null,
+      durationMs: null,
+      coverPath: null,
+      dateAddedMs: 0,
+      updatedAtMs: 0,
+      deleted: false,
+    );
+
+    final item = songToMediaItem(song);
+
+    expect(item.extras?['songId'], song.id);
+  });
+}