task-15(壳层/路由): 实现页面标题与面包屑规则

title-breadcrumb 提供 resolveDocumentTitle('页面 - 数富AI')、updateDocumentTitle
(非浏览器静默)、crumbsForActiveRoute(分组->页面)；AdminLayout 用 watch 同步
document.title 并在层级>1 时渲染 el-breadcrumb。

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

@@ -1,5 +1,5 @@
 <script setup lang="ts">
-import { computed, ref } from 'vue'
+import { computed, ref, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 import { useAdminSessionStore } from '@/stores/admin-session'
@@ -13,6 +13,7 @@ import {
 } from '@/layout/empty-state'
 import GlobalErrorContainer from '@/layout/GlobalErrorContainer.vue'
 import { initialCollapsed, toggleCollapsed, writeCollapsePreference } from '@/layout/sidebar-collapse'
+import { crumbsForActiveRoute, resolveDocumentTitle, updateDocumentTitle } from '@/layout/title-breadcrumb'
 
 const route = useRoute()
 const router = useRouter()
@@ -40,8 +41,15 @@ const openedKeys = computed(() => groupKeysForActive(menuEntries.value, route.pa
 const emptyMenu = computed(() => shouldShowEmptyMenu(menuEntries.value.length > 0, session.loading, session.initialized))
 const pageTitle = computed(() => pageTitleOf(route.meta.title))
 const userVm = computed(() => topbarUserOf(session.user))
+const breadcrumbs = computed(() => crumbsForActiveRoute(session.menuTree, route.path))
 const logoUrl = joinAdminPath('assets', 'logo.jpg')
 
+watch(
+  pageTitle,
+  (title) => updateDocumentTitle(resolveDocumentTitle(title), typeof document !== 'undefined' ? document : null),
+  { immediate: true },
+)
+
 async function signOut() {
   try {
     await session.signOut()
@@ -101,6 +109,9 @@ async function signOut() {
         </div>
       </header>
       <main class="admin-content">
+        <el-breadcrumb v-if="breadcrumbs.length > 1" class="admin-breadcrumb" separator="/">
+          <el-breadcrumb-item v-for="crumb in breadcrumbs" :key="crumb.key">{{ crumb.name }}</el-breadcrumb-item>
+        </el-breadcrumb>
         <GlobalErrorContainer />
         <el-alert v-if="session.error" :title="session.error" type="error" show-icon :closable="false" />
         <el-empty