feat(flutter): playlists list + detail screens

- models/playlist.dart (Playlist, PlaylistTrack, PlaylistDetail)
- api/endpoints/playlists.dart (list with kind=user|system|all, get detail)
- playlists/playlists_provider.dart (Riverpod family providers)
- playlists/playlists_list_screen.dart (with system-variant pill)
- playlists/playlist_detail_screen.dart (header + Play button + rows)
- /playlists + /playlists/:id routes wired
- Home AppBar: queue_music icon next to Search

Tracks with track_id=null render greyed-out + struck-through (matches
web's PlaylistTrackRow behavior). Tap a row plays from that position;
top-level Play button plays the full playable subset from track 0.

flutter_client/lib/shared/routing.dart

@@ -10,6 +10,8 @@ import '../library/artist_detail_screen.dart';
 import '../library/home_screen.dart';
 import '../player/now_playing_screen.dart';
 import '../player/player_bar.dart';
+import '../playlists/playlist_detail_screen.dart';
+import '../playlists/playlists_list_screen.dart';
 import '../search/search_screen.dart';
 import 'widgets/version_gate.dart';
 
@@ -53,6 +55,11 @@ GoRouter buildRouter(Ref ref) {
           ),
           GoRoute(path: '/now-playing', builder: (_, __) => const NowPlayingScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
+          GoRoute(path: '/playlists', builder: (_, __) => const PlaylistsListScreen()),
+          GoRoute(
+            path: '/playlists/:id',
+            builder: (_, s) => PlaylistDetailScreen(id: s.pathParameters['id']!),
+          ),
         ],
       ),
     ],