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/models/my_profile.dart

@@ -0,0 +1,49 @@
+/// Mirrors web/src/lib/api/me.ts MyProfile. The `display_name` and
+/// `email` are nullable — server returns null when the user hasn't
+/// set them yet (registration only requires a username).
+class MyProfile {
+  const MyProfile({
+    required this.id,
+    required this.username,
+    this.displayName,
+    this.email,
+    required this.isAdmin,
+  });
+
+  final String id;
+  final String username;
+  final String? displayName;
+  final String? email;
+  final bool isAdmin;
+
+  factory MyProfile.fromJson(Map<String, dynamic> j) => MyProfile(
+        id: j['id'] as String? ?? '',
+        username: j['username'] as String? ?? '',
+        displayName: j['display_name'] as String?,
+        email: j['email'] as String?,
+        isAdmin: j['is_admin'] as bool? ?? false,
+      );
+}
+
+/// Mirrors web/src/lib/api/listenbrainz.ts LBStatus.
+class ListenBrainzStatus {
+  const ListenBrainzStatus({
+    required this.enabled,
+    required this.tokenSet,
+    this.lastScrobbledAt,
+  });
+
+  final bool enabled;
+  /// True when the user has stored a token (token itself is never read
+  /// back from the server). UI uses this to show "token saved" vs
+  /// "no token" without exposing the value.
+  final bool tokenSet;
+  final String? lastScrobbledAt;
+
+  factory ListenBrainzStatus.fromJson(Map<String, dynamic> j) =>
+      ListenBrainzStatus(
+        enabled: j['enabled'] as bool? ?? false,
+        tokenSet: j['token_set'] as bool? ?? false,
+        lastScrobbledAt: j['last_scrobbled_at'] as String?,
+      );
+}