task-18(壳层/路由): 实现壳层键盘焦点与可访问性语义

AdminLayout 增加 role=navigation 侧边栏地标、aria-expanded/aria-label 折叠按钮、
#admin-content 主区锚点与“跳到主内容”skip-link；main.css 提供 :focus-visible 焦点环
与 skip-link 聚焦样式；e2e 8 用例用键盘回车切换折叠并断言 ARIA 语义。

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

@@ -79,7 +79,8 @@ async function signOut() {
 
 <template>
   <div class="admin-shell" :class="{ 'is-collapsed': collapsed }">
-    <aside class="admin-sidebar">
+    <a class="skip-link" href="#admin-content">跳到主内容</a>
+    <aside class="admin-sidebar" role="navigation" aria-label="侧边栏菜单">
       <div class="admin-brand">
         <img :src="logoUrl" alt="数富AI" class="admin-brand-logo" />
         <div v-if="!collapsed" class="admin-brand-copy">
@@ -107,7 +108,13 @@ async function signOut() {
         <el-empty v-if="emptyMenu" :description="EMPTY_MENU_TITLE" :image-size="72" />
       </el-scrollbar>
 
-      <button class="sidebar-collapse" type="button" @click="toggleSidebar">
+      <button
+        class="sidebar-collapse"
+        type="button"
+        :aria-expanded="!collapsed"
+        aria-label="切换侧边栏"
+        @click="toggleSidebar"
+      >
         {{ collapsed ? '展开菜单' : '收起菜单' }}
       </button>
     </aside>
@@ -126,7 +133,7 @@ async function signOut() {
           <el-button text type="danger" @click="signOut">退出登录</el-button>
         </div>
       </header>
-      <main class="admin-content">
+      <main id="admin-content" 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>