feat(flutter): adopt MainAppBarActions in Search, Discover, Playlists, Settings

Search keeps its conditional clear-text button and gains the shared
nav widget after it. Discover/Playlists/Settings get an actions row
they didn't have before. The kebab is now reachable from every
top-level screen, which is the prerequisite for the Admin entry.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/settings/settings_screen.dart

@@ -7,6 +7,7 @@ import '../api/endpoints/settings.dart';
 import '../api/errors.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/my_profile.dart';
+import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
 
 final _settingsApiProvider = FutureProvider<SettingsApi>((ref) async {
@@ -37,6 +38,7 @@ class SettingsScreen extends ConsumerWidget {
           onPressed: () => context.pop(),
         ),
         title: Text('Settings', style: TextStyle(color: fs.parchment)),
+        actions: const [MainAppBarActions(currentRoute: '/settings')],
       ),
       body: ListView(
         padding: const EdgeInsets.symmetric(vertical: 8),