task-14(壳层/路由): 实现侧边栏折叠与窄屏行为及偏好持久化

sidebar-collapse 提供阈值/宽高常量、toggle、localStorage 偏好读写(失败容忍)、
initialCollapsed(持久化优先，窄屏自动收起)；AdminLayout 接入并持久化折叠状态。

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

@@ -12,11 +12,27 @@ import {
   shouldShowEmptyMenu,
 } from '@/layout/empty-state'
 import GlobalErrorContainer from '@/layout/GlobalErrorContainer.vue'
+import { initialCollapsed, toggleCollapsed, writeCollapsePreference } from '@/layout/sidebar-collapse'
 
 const route = useRoute()
 const router = useRouter()
 const session = useAdminSessionStore()
-const collapsed = ref(false)
+const collapsed = ref(readInitialCollapse())
+
+function readInitialCollapse(): boolean {
+  const storage = typeof window !== 'undefined' ? window.localStorage : null
+  const width = typeof window !== 'undefined' ? window.innerWidth : 0
+  return initialCollapsed(width, storage)
+}
+
+function setCollapsed(value: boolean): void {
+  collapsed.value = value
+  if (typeof window !== 'undefined') writeCollapsePreference(window.localStorage, value)
+}
+
+function toggleSidebar(): void {
+  setCollapsed(toggleCollapsed(collapsed.value))
+}
 
 const activePath = computed(() => route.path)
 const menuEntries = computed(() => toSidebarEntries(session.menuTree))
@@ -65,7 +81,7 @@ async function signOut() {
         <el-empty v-if="emptyMenu" :description="EMPTY_MENU_TITLE" :image-size="72" />
       </el-scrollbar>
 
-      <button class="sidebar-collapse" type="button" @click="collapsed = !collapsed">
+      <button class="sidebar-collapse" type="button" @click="toggleSidebar">
         {{ collapsed ? '展开菜单' : '收起菜单' }}
       </button>
     </aside>