task-10(壳层/路由): 实现页面级异步组件加载边界

lazy-config 暴露 loading 延迟/超时/文案与懒加载判定；lazy-page 用
defineAsyncComponent 提供 loading/error 组件，router 对所有注册路由统一包裹；
main.css 增加 .page-loading/.page-error 兜底样式。

admin-frontend-vue/src/router/index.ts

@@ -1,9 +1,10 @@
-import { createRouter, createWebHistory } from 'vue-router'
+import { createRouter, createWebHistory, type RouteRecordRaw } from 'vue-router'
 import type { AdminMenuNode } from '@/types/admin'
 import AdminLayout from '@/layout/AdminLayout.vue'
 import { useAdminSessionStore } from '@/stores/admin-session'
 import { APP_BASE_PATH } from '@/config/app'
 import { adminRouteRecords } from './routes'
+import { defineLazyPage } from './lazy-page'
 
 const router = createRouter({
   history: createWebHistory(APP_BASE_PATH),
@@ -11,7 +12,10 @@ const router = createRouter({
     {
       path: '/',
       component: AdminLayout,
-      children: adminRouteRecords,
+      children: adminRouteRecords.map((record) => ({
+        ...record,
+        component: defineLazyPage(record.component as () => Promise<{ default: unknown }>),
+      })) as RouteRecordRaw[],
     },
   ],
 })