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/shared/routing.dart

@@ -15,6 +15,7 @@ import '../player/queue_screen.dart';
 import '../playlists/playlist_detail_screen.dart';
 import '../playlists/playlists_list_screen.dart';
 import '../search/search_screen.dart';
+import '../settings/settings_screen.dart';
 import 'widgets/version_gate.dart';
 
 /// Exposed as a Provider so its single argument is a real `Ref` (the
@@ -59,6 +60,7 @@ GoRouter buildRouter(Ref ref) {
           GoRoute(path: '/queue', builder: (_, __) => const QueueScreen()),
           GoRoute(path: '/search', builder: (_, __) => const SearchScreen()),
           GoRoute(path: '/library', builder: (_, __) => const LibraryScreen()),
+          GoRoute(path: '/settings', builder: (_, __) => const SettingsScreen()),
           GoRoute(path: '/playlists', builder: (_, __) => const PlaylistsListScreen()),
           GoRoute(
             path: '/playlists/:id',