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/likes/like_button.dart

@@ -0,0 +1,36 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import '../api/endpoints/likes.dart';
+import '../theme/theme_extension.dart';
+import 'likes_provider.dart';
+
+class LikeButton extends ConsumerWidget {
+  const LikeButton({
+    required this.kind,
+    required this.id,
+    this.size = 22,
+    super.key,
+  });
+  final LikeKind kind;
+  final String id;
+  final double size;
+
+  @override
+  Widget build(BuildContext context, WidgetRef ref) {
+    final fs = Theme.of(context).extension<FabledSwordTheme>()!;
+    final state = ref.watch(likedIdsProvider);
+    final liked = state.maybeWhen(
+      data: (s) => s.has(kind, id),
+      orElse: () => false,
+    );
+    return IconButton(
+      icon: Icon(
+        liked ? Icons.favorite : Icons.favorite_border,
+        color: liked ? fs.accent : fs.ash,
+        size: size,
+      ),
+      onPressed: () => ref.read(likedIdsProvider.notifier).toggle(kind, id),
+    );
+  }
+}