fix(flutter): PlaylistsApi.list parses {owned, public} envelope

The server has always returned an enveloped response from GET
/api/playlists; the existing client decoded it as a flat List which
fails at runtime against the actual Map shape. Existing playlists
list screen would have been broken on any production instance.

list() now returns PlaylistsList { owned, public }. The existing
list screen consumes lists.all (owned + public flattened) — same
visible output as the previous flat-list assumption.

This unblocks the home parity slice which needs the same endpoint
for the new Playlists row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/api/endpoints/playlists.dart

@@ -2,23 +2,42 @@ import 'package:dio/dio.dart';
 
 import '../../models/playlist.dart';
 
+/// Wire shape for GET /api/playlists. Server splits owned vs. public so
+/// the UI can present them as different sections; we preserve that split
+/// to give the integrations page room to grow.
+class PlaylistsList {
+  const PlaylistsList({required this.owned, required this.public});
+  final List<Playlist> owned;
+  final List<Playlist> public;
+
+  factory PlaylistsList.empty() =>
+      const PlaylistsList(owned: [], public: []);
+
+  /// Concatenated view for callers that don't care about the split.
+  List<Playlist> get all => [...owned, ...public];
+}
+
 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>>(
+  /// GET /api/playlists?kind=user|system|all. Server returns
+  /// `{"owned": [...], "public": [...]}`. Owned is the caller's own
+  /// playlists, filtered by the kind param (default "user"). Public
+  /// is other users' shared playlists; not filtered by kind.
+  Future<PlaylistsList> list({String kind = 'user'}) async {
+    final r = await _dio.get<Map<String, 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);
+    final body = r.data ?? const <String, dynamic>{};
+    List<Playlist> parse(String key) {
+      final raw = (body[key] as List?) ?? const [];
+      return raw
+          .map((e) => Playlist.fromJson((e as Map).cast<String, dynamic>()))
+          .toList(growable: false);
+    }
+    return PlaylistsList(owned: parse('owned'), public: parse('public'));
   }
 
   Future<PlaylistDetail> get(String id) async {

flutter_client/lib/playlists/playlists_list_screen.dart

@@ -31,7 +31,8 @@ class PlaylistsListScreen extends ConsumerWidget {
         error: (e, _) => Center(
           child: Text('$e', style: TextStyle(color: fs.error)),
         ),
-        data: (items) {
+        data: (lists) {
+          final items = lists.all;
           if (items.isEmpty) {
             return Center(
               child: Text(

flutter_client/lib/playlists/playlists_provider.dart

@@ -9,7 +9,7 @@ final playlistsApiProvider = FutureProvider<PlaylistsApi>((ref) async {
 });
 
 final playlistsListProvider =
-    FutureProvider.family<List<Playlist>, String>((ref, kind) async {
+    FutureProvider.family<PlaylistsList, String>((ref, kind) async {
   return (await ref.watch(playlistsApiProvider.future)).list(kind: kind);
 });