feat(flutter/likes): LikeButton with optimistic toggle + rollback

LikedIdsController loads /api/likes/ids once and mutates the local set
on toggle. Failed mutations roll the set back so the icon never lies.
Wired into ArtistDetail header, AlbumDetail header, and per-track in
the album track list.

flutter_client/lib/api/endpoints/likes.dart

@@ -0,0 +1,40 @@
+import 'package:dio/dio.dart';
+
+enum LikeKind { artist, album, track }
+
+extension LikeKindPath on LikeKind {
+  String get path => switch (this) {
+        LikeKind.artist => 'artists',
+        LikeKind.album => 'albums',
+        LikeKind.track => 'tracks',
+      };
+}
+
+class LikesApi {
+  LikesApi(this._dio);
+  final Dio _dio;
+
+  Future<void> like(LikeKind kind, String id) async {
+    await _dio.post<void>('/api/likes/${kind.path}/$id');
+  }
+
+  Future<void> unlike(LikeKind kind, String id) async {
+    await _dio.delete<void>('/api/likes/${kind.path}/$id');
+  }
+
+  /// Returns sets of {artists, albums, tracks} the user has liked.
+  /// Server response keys (verified in internal/api/likes.go
+  /// `likedIDsResponse`): `artist_ids`, `album_ids`, `track_ids`.
+  Future<({Set<String> artists, Set<String> albums, Set<String> tracks})>
+      ids() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/likes/ids');
+    final body = r.data ?? const <String, dynamic>{};
+    Set<String> set(String key) =>
+        ((body[key] as List?) ?? const []).map((e) => e.toString()).toSet();
+    return (
+      artists: set('artist_ids'),
+      albums: set('album_ids'),
+      tracks: set('track_ids'),
+    );
+  }
+}