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/api/endpoints/playlists.dart

@@ -0,0 +1,28 @@
+import 'package:dio/dio.dart';
+
+import '../../models/playlist.dart';
+
+class PlaylistsApi {
+  PlaylistsApi(this._dio);
+  final Dio _dio;
+
+  /// GET /api/playlists?kind=user|system|all. The server's default kind
+  /// is "user" — passing "all" returns user playlists + system mixes.
+  /// "system" returns just for-you / discover. The mobile UI defaults
+  /// to "all" so users see their generated mixes alongside their own.
+  Future<List<Playlist>> list({String kind = 'all'}) async {
+    final r = await _dio.get<List<dynamic>>(
+      '/api/playlists',
+      queryParameters: {'kind': kind},
+    );
+    final raw = r.data ?? const [];
+    return raw
+        .map((e) => Playlist.fromJson((e as Map).cast<String, dynamic>()))
+        .toList(growable: false);
+  }
+
+  Future<PlaylistDetail> get(String id) async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/playlists/$id');
+    return PlaylistDetail.fromJson(r.data ?? const {});
+  }
+}