fix: ConfirmDialog确认后弹框不关闭(9页面) + 登录页顶部渐变线移除

- 9个页面 onConfirm 回调添加 setDeleteConfirm(null) 关闭弹框
  - InstancePage/InstanceDetailPage/InstanceGroupPage/RedisDetailPage
  - UsersPage/AnnouncementPage/NotificationPage/NotificationsPage/WorkflowPage
- 白日梦境(NeonCityView) + 云石纹理(GradientMeshView) 移除顶部蓝色渐变线

frontend/src/pages/InstanceDetailPage.tsx

@@ -268,7 +268,7 @@ export default function InstanceDetailPage() {
         title={deleteConfirm?.msg || '确认操作'}
         variant="danger"
         confirmText="确定"
-        onConfirm={() => { deleteConfirm?.onConfirm(); }}
+        onConfirm={() => { deleteConfirm?.onConfirm(); setDeleteConfirm(null); }}
         onCancel={() => setDeleteConfirm(null)}
       />
     </div>