fix(client): NotificationBell.onTap 恢复 required 非空(对齐规范)

- notification_bell.dart: VoidCallback? → VoidCallback（字段非空）
- content_top_bar.dart: 在 null-guard 内用 onBell! 传给 NotificationBell
- flutter analyze: No issues found

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/lib/widgets/content_top_bar.dart

@@ -63,7 +63,7 @@ class ContentTopBar extends ConsumerWidget {
         ),
         if (onBell != null) ...[
           const SizedBox(width: 12),
-          NotificationBell(onTap: onBell, hasUnread: true),
+          NotificationBell(onTap: onBell!, hasUnread: true),
         ],
         if (showThemeToggle) ...[
           const SizedBox(width: 12),

client/lib/widgets/notification_bell.dart

@@ -10,7 +10,7 @@ import 'pangolin_icons.dart';
 class NotificationBell extends ConsumerWidget {
   const NotificationBell({super.key, required this.onTap, this.hasUnread = false});
 
-  final VoidCallback? onTap;
+  final VoidCallback onTap;
   final bool hasUnread;
 
   @override