feat(flutter): settings screen + 4-icon home AppBar

- models/my_profile.dart (MyProfile + ListenBrainzStatus)
- api/endpoints/settings.dart (profile / password / listenbrainz CRUD)
- settings/settings_screen.dart with three sections:
  - Profile: display_name + email, Save profile
  - Password: current + new + confirm, validates >=8 + match
  - ListenBrainz: token paste + scrobble-enabled toggle
- /settings route + Settings icon on home AppBar
- Home AppBar now: Library + Playlists + Search + Settings (was just Search)

flutter_client/lib/api/endpoints/settings.dart

@@ -0,0 +1,57 @@
+import 'package:dio/dio.dart';
+
+import '../../models/my_profile.dart';
+
+class SettingsApi {
+  SettingsApi(this._dio);
+  final Dio _dio;
+
+  Future<MyProfile> getProfile() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/me');
+    return MyProfile.fromJson(r.data ?? const {});
+  }
+
+  /// Pass only the fields you want to change; server merges. Empty
+  /// strings clear; null leaves the existing value alone.
+  Future<MyProfile> updateProfile({String? displayName, String? email}) async {
+    final body = <String, dynamic>{};
+    if (displayName != null) body['display_name'] = displayName;
+    if (email != null) body['email'] = email;
+    final r = await _dio.put<Map<String, dynamic>>(
+      '/api/me/profile',
+      data: body,
+    );
+    return MyProfile.fromJson(r.data ?? const {});
+  }
+
+  Future<void> changePassword({
+    required String current,
+    required String next,
+  }) async {
+    await _dio.put<void>('/api/me/password', data: {
+      'current_password': current,
+      'new_password': next,
+    });
+  }
+
+  Future<ListenBrainzStatus> getListenBrainz() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/me/listenbrainz');
+    return ListenBrainzStatus.fromJson(r.data ?? const {});
+  }
+
+  Future<ListenBrainzStatus> setListenBrainzToken(String token) async {
+    final r = await _dio.put<Map<String, dynamic>>(
+      '/api/me/listenbrainz',
+      data: {'token': token},
+    );
+    return ListenBrainzStatus.fromJson(r.data ?? const {});
+  }
+
+  Future<ListenBrainzStatus> setListenBrainzEnabled(bool enabled) async {
+    final r = await _dio.put<Map<String, dynamic>>(
+      '/api/me/listenbrainz',
+      data: {'enabled': enabled},
+    );
+    return ListenBrainzStatus.fromJson(r.data ?? const {});
+  }
+}