task-12(壳层/路由): 实现路由级错误页面(404 兜底)

新增 src/pages/error/NotFoundPage.vue（el-result + 返回首页），router 在业务路由
之后注册 :pathMatch(.*)* 兜底（无 menuKey，不参与权限门禁），保留壳层 chrome；
main.css 增加 .not-found 布局。

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

@@ -1,6 +1,7 @@
 import { createRouter, createWebHistory, type RouteRecordRaw } from 'vue-router'
 import type { AdminMenuNode } from '@/types/admin'
 import AdminLayout from '@/layout/AdminLayout.vue'
+import NotFoundPage from '@/pages/error/NotFoundPage.vue'
 import { useAdminSessionStore } from '@/stores/admin-session'
 import { APP_BASE_PATH } from '@/config/app'
 import { adminRouteRecords } from './routes'
@@ -12,10 +13,13 @@ const router = createRouter({
     {
       path: '/',
       component: AdminLayout,
-      children: adminRouteRecords.map((record) => ({
-        ...record,
-        component: defineLazyPage(record.component as () => Promise<{ default: unknown }>),
-      })) as RouteRecordRaw[],
+      children: [
+        ...adminRouteRecords.map((record) => ({
+          ...record,
+          component: defineLazyPage(record.component as () => Promise<{ default: unknown }>),
+        })),
+        { path: ':pathMatch(.*)*', component: NotFoundPage, meta: { title: '页面不存在' } },
+      ] as RouteRecordRaw[],
     },
   ],
 })