feat(flutter): queue screen + skipToQueueItem support

- audio_handler: override skipToQueueItem(index) -> _player.seek(zero, index: i)
- player/queue_screen.dart: list of MediaItems with current track highlighted
  (left accent border + Now Playing badge), tap-to-jump on non-current rows
- /queue route + queue_music icon on /now-playing AppBar

Reorder + remove deferred for v1.

flutter_client/lib/shared/routing.dart

@@ -11,6 +11,7 @@ import '../library/home_screen.dart';
 import '../library/library_screen.dart';
 import '../player/now_playing_screen.dart';
 import '../player/player_bar.dart';
+import '../player/queue_screen.dart';
 import '../playlists/playlist_detail_screen.dart';
 import '../playlists/playlists_list_screen.dart';
 import '../search/search_screen.dart';
@@ -55,6 +56,7 @@ GoRouter buildRouter(Ref ref) {
             builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
           ),
           GoRoute(path: '/now-playing', builder: (_, __) => const NowPlayingScreen()),
+          GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
           GoRoute(path: '/library', builder: (_, __) => const LibraryScreen()),
           GoRoute(path: '/playlists', builder: (_, __) => const PlaylistsListScreen()),