refactor: AlertCenter拆分(723→390行)+修复TopologyView类型错误

- AlertCenter.tsx: 723→390行，提取类型/helpers/渠道Tab到子组件
- alert-center-helpers.tsx: 新建(127行)，共享Alert/AlertRule/NotificationChannel类型+mapAlert+badge helpers
- AlertChannelDialog.tsx: 新建(121行)，通知渠道创建/编辑对话框(自包含useEffect同步)
- AlertChannelsTab.tsx: 新建(152行)，通知渠道Tab自包含(状态+API+渲染+CRUD)
- AlertCenterDetail.tsx: 去重getSeverityBadge，统一从helpers导入
- TopologyView.tsx: 修复editForm.type类型推断(string→TopologyNode['type'])

frontend/src/components/monitor/alert-center/AlertCenterDetail.tsx

@@ -1,26 +1,11 @@
-import {
-  Check, ChevronRight, AlertCircle, AlertTriangle, Info,
-} from 'lucide-react';
+import { Check, ChevronRight } from 'lucide-react';
 import { Button } from '@/components/ui/button';
 import { Badge } from '@/components/ui/badge';
 import { Label } from '@/components/ui/label';
 import {
   Dialog, DialogContent, DialogHeader, DialogTitle, DialogDescription, DialogFooter,
 } from '@/components/ui/dialog';
-import type { Alert } from './AlertCenter';
-
-function getSeverityBadge(severity: string) {
-  switch (severity) {
-    case 'critical':
-      return <Badge variant="destructive" className="gap-1"><AlertCircle className="w-3 h-3" />严重</Badge>;
-    case 'warning':
-      return <Badge variant="default" className="bg-orange-500 gap-1"><AlertTriangle className="w-3 h-3" />警告</Badge>;
-    case 'info':
-      return <Badge variant="default" className="bg-blue-500 gap-1"><Info className="w-3 h-3" />信息</Badge>;
-    default:
-      return <Badge variant="secondary">未知</Badge>;
-  }
-}
+import { type Alert, getSeverityBadge } from './alert-center-helpers'; // Logan.Li — 统一从 helpers 导入
 
 export function AlertDetailDialog({
   alert,