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/playlists/playlists_list_screen.dart

@@ -3,6 +3,7 @@ import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
 import '../models/playlist.dart';
+import '../shared/widgets/main_app_bar_actions.dart';
 import '../theme/theme_extension.dart';
 import 'playlists_provider.dart';
 
@@ -22,6 +23,7 @@ class PlaylistsListScreen extends ConsumerWidget {
         backgroundColor: fs.obsidian,
         elevation: 0,
         title: Text('Playlists', style: TextStyle(color: fs.parchment)),
+        actions: const [MainAppBarActions(currentRoute: '/playlists')],
       ),
       body: list.when(
         loading: () => const Center(child: CircularProgressIndicator()),