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/discover/discover_screen.dart

@@ -7,6 +7,7 @@ import '../api/endpoints/discover.dart';
 import '../api/errors.dart';
 import '../library/library_providers.dart' show dioProvider;
 import '../models/lidarr.dart';
+import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
 
 final _discoverApiProvider = FutureProvider<DiscoverApi>((ref) async {
@@ -87,6 +88,7 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
           onPressed: () => context.pop(),
         ),
         title: Text('Discover', style: TextStyle(color: fs.parchment)),
+        actions: const [MainAppBarActions(currentRoute: '/discover')],
       ),
       body: Column(children: [
         Padding(