feat(flutter/player): NowPlaying full-screen view with scrubber

Reached by tapping the PlayerBar. Reads media + playback streams; seek
dispatches through the audio handler. Swipe-down dismiss is a stand-in
chevron-down AppBar leading icon for v1.

flutter_client/lib/shared/routing.dart

@@ -8,6 +8,7 @@ import '../auth/server_url_screen.dart';
 import '../library/album_detail_screen.dart';
 import '../library/artist_detail_screen.dart';
 import '../library/home_screen.dart';
+import '../player/now_playing_screen.dart';
 import '../player/player_bar.dart';
 import 'widgets/version_gate.dart';
 
@@ -43,6 +44,7 @@ GoRouter buildRouter(Ref ref) {
             path: '/albums/:id',
             builder: (_, s) => AlbumDetailScreen(id: s.pathParameters['id']!),
           ),
+          GoRoute(path: '/now-playing', builder: (_, __) => const NowPlayingScreen()),
         ],
       ),
     ],