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/search/search_screen.dart

@@ -7,6 +7,7 @@ import '../library/widgets/artist_card.dart';
 import '../library/widgets/track_row.dart';
 import '../models/search_response.dart';
 import '../player/player_provider.dart';
+import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
 import 'search_provider.dart';
 
@@ -73,6 +74,7 @@ class _SearchScreenState extends ConsumerState<SearchScreen> {
                 _focus.requestFocus();
               },
             ),
+          const MainAppBarActions(currentRoute: '/search'),
         ],
       ),
       body: results.when(