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/library/artist_detail_screen.dart

@@ -2,6 +2,8 @@ import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
+import '../api/endpoints/likes.dart';
+import '../likes/like_button.dart';
 import '../models/album.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
@@ -50,7 +52,7 @@ class ArtistDetailScreen extends ConsumerWidget {
                   onPressed: () { /* play wired in Task 18 */ },
                 ),
               ),
-              // LikeButton goes here in Task 16.
+              LikeButton(kind: LikeKind.artist, id: a.id, size: 28),
             ]),
           ),
           const Padding(