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/album_detail_screen.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
+import '../api/endpoints/likes.dart';
+import '../likes/like_button.dart';
 import '../theme/theme_extension.dart';
 import 'library_providers.dart';
 import 'widgets/track_row.dart';
@@ -43,11 +45,15 @@ class AlbumDetailScreen extends ConsumerWidget {
                       onPressed: () { /* play wired in Task 18 */ },
                     ),
                   ),
-                  // LikeButton goes here in Task 16.
+                  LikeButton(kind: LikeKind.album, id: r.album.id, size: 28),
                 ]),
               ),
               for (final t in r.tracks)
-                TrackRow(track: t, onTap: () { /* play wired in Task 18 */ }),
+                TrackRow(
+                  track: t,
+                  onTap: () { /* play wired in Task 18 */ },
+                  trailing: LikeButton(kind: LikeKind.track, id: t.id),
+                ),
             ]),
           ),
     );

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(