fix(flutter): clear analyze --fatal-infos errors from admin parity slice

CI caught six issues against the new admin slice:

- AsyncValue<T> in this Riverpod 3.3.1 codebase exposes `.value`
  (returns T?), not `.valueOrNull`. Switched the three admin readers
  to match the established convention (player_bar, queue_screen,
  now_playing_screen all use `.value`).
- home_screen.dart still has ctx.push calls in _albumsRow / _artistsRow
  (carousel tap handlers) — restored the go_router import that
  Task 2 over-eagerly removed.
- admin_user_edit_sheet.dart had a single-line `if (!await ...) return;`
  that violated curly_braces_in_flow_control_structures. Wrapped in
  braces.
- admin_quarantine_item.dart doc comment had `<top>` placeholders that
  the analyzer flagged as unintended HTML. Rephrased without angle
  brackets.

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

flutter_client/lib/shared/widgets/main_app_bar_actions.dart

@@ -17,7 +17,7 @@ class MainAppBarActions extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final fs = Theme.of(context).extension<FabledSwordTheme>()!;
-    final user = ref.watch(authControllerProvider).valueOrNull;
+    final user = ref.watch(authControllerProvider).value;
     final isAdmin = user?.isAdmin ?? false;
 
     return Row(