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/library/home_screen.dart

@@ -29,6 +29,11 @@ class HomeScreen extends ConsumerWidget {
         elevation: 0,
         title: Text('Minstrel', style: TextStyle(color: fs.parchment)),
         actions: [
+          IconButton(
+            icon: Icon(Icons.library_music, color: fs.parchment),
+            tooltip: 'Library',
+            onPressed: () => context.push('/library'),
+          ),
           IconButton(
             icon: Icon(Icons.queue_music, color: fs.parchment),
             tooltip: 'Playlists',
@@ -39,6 +44,11 @@ class HomeScreen extends ConsumerWidget {
             tooltip: 'Search',
             onPressed: () => context.push('/search'),
           ),
+          IconButton(
+            icon: Icon(Icons.settings, color: fs.parchment),
+            tooltip: 'Settings',
+            onPressed: () => context.push('/settings'),
+          ),
         ],
       ),
       body: SafeArea(