后台统一状态提示：共享 toast 模块（ToastStack 挂后台布局与登录页），登录成功/失败、文章/标签/作品/文件/评论/设置操作全部有反馈，替换掉原生 alert；编辑器局部 toast 并入共享实现

frontend/src/admin/AdminLayout.vue

@@ -1,5 +1,6 @@
 <script setup>
 import { computed, onMounted, onBeforeUnmount, ref, watch } from 'vue'
+import ToastStack from './ToastStack.vue'
 import { useRoute, useRouter, RouterLink, RouterView } from 'vue-router'
 import { checkAuth } from './auth'
 import { adminApi, session } from '../api'
@@ -96,6 +97,7 @@ watch(() => route.path, () => loadCounts())
 
 <template>
   <div v-if="ready" class="admin-shell">
+    <ToastStack />
     <aside class="admin-side">
       <div class="brand">
         <RouterLink to="/admin" class="name">{{ site.site_title || 'ONE' }}</RouterLink>