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/player/now_playing_screen.dart

@@ -1,5 +1,6 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:go_router/go_router.dart';
 
 import '../theme/theme_extension.dart';
 import 'player_provider.dart';
@@ -28,6 +29,13 @@ class NowPlayingScreen extends ConsumerWidget {
           icon: Icon(Icons.expand_more, color: fs.parchment),
           onPressed: () => Navigator.of(context).pop(),
         ),
+        actions: [
+          IconButton(
+            icon: Icon(Icons.queue_music, color: fs.parchment),
+            tooltip: 'Queue',
+            onPressed: () => GoRouter.of(context).push('/queue'),
+          ),
+        ],
       ),
       body: SafeArea(
         child: Padding(