task-5(壳层/路由): 定义顶部栏用户信息与页面标题模型

新增 src/layout/topbar-model.ts（pageTitleOf 缺省回落 + topbarUserOf 空态视图模型），
AdminLayout 改用模型渲染标题/用户名/角色，删除模板内联缺省逻辑；8 用例覆盖标题
空值、用户缺字段、非法输入等。

admin-frontend-vue/src/layout/AdminLayout.vue

@@ -4,6 +4,7 @@ import { useRoute, useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 import { useAdminSessionStore } from '@/stores/admin-session'
 import { joinAdminPath } from '@/config/app'
+import { pageTitleOf, topbarUserOf } from '@/layout/topbar-model'
 import type { AdminMenuNode } from '@/types/admin'
 
 const route = useRoute()
@@ -13,7 +14,8 @@ const collapsed = ref(false)
 
 const activePath = computed(() => route.path)
 const groups = computed(() => session.menuTree.filter((node) => node.children?.length))
-const pageTitle = computed(() => String(route.meta.title || '管理后台'))
+const pageTitle = computed(() => pageTitleOf(route.meta.title as string | undefined))
+const userVm = computed(() => topbarUserOf(session.user))
 const logoUrl = joinAdminPath('assets', 'logo.jpg')
 
 function menuPath(node: AdminMenuNode): string {
@@ -85,8 +87,8 @@ async function signOut() {
         </div>
         <div class="admin-user">
           <div class="admin-user-meta">
-            <strong>{{ session.user?.username || '当前用户' }}</strong>
-            <span>{{ session.user?.role || '' }}</span>
+            <strong>{{ userVm.username }}</strong>
+            <span v-if="userVm.role">{{ userVm.role }}</span>
           </div>
           <el-button text type="danger" @click="signOut">退出登录</el-button>
         </div>