feat(notif): 通知占位屏 + 移动顶栏铃铛(Spec ③接口占位)

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

client/lib/shell/desktop_shell.dart

@@ -10,6 +10,7 @@ import '../screens/connect_page.dart';
 import '../screens/contact_page.dart';
 import '../screens/invite_page.dart';
 import '../screens/nodes_page.dart';
+import '../screens/notifications_page.dart';
 import '../screens/orders_page.dart';
 import '../screens/payment_page.dart';
 import '../screens/purchase_page.dart';
@@ -91,8 +92,7 @@ class DesktopShell extends ConsumerWidget {
         case NavView.invite:
           return InviteScreen(t: t, embedded: true);
         case NavView.notifications:
-          // TODO(task-6): 通知屏尚未接线,占位回落账户页,避免非穷尽 switch 编译失败。
-          return const AccountPage(isWide: true);
+          return NotificationsScreen(t: t, embedded: true);
       }
     }