task-13(壳层/路由): 实现全局请求错误提示容器

error-bus 提供框架无关的订阅式错误总线：notify/dismiss/clear、负载归一化、
3 条上限截断、订阅返回取消函数防泄漏；GlobalErrorContainer 订阅并渲染可关闭
错误条，挂在 AdminLayout 内容区顶部。

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

@@ -0,0 +1,35 @@
+<script setup lang="ts">
+import { onBeforeUnmount, onMounted, ref } from 'vue'
+import {
+  dismissGlobalError,
+  subscribeGlobalError,
+  type GlobalErrorItem,
+} from '@/layout/error-bus'
+
+const errors = ref<GlobalErrorItem[]>([])
+let unsubscribe: (() => void) | null = null
+
+onMounted(() => {
+  unsubscribe = subscribeGlobalError((list) => {
+    errors.value = list
+  })
+})
+onBeforeUnmount(() => {
+  unsubscribe?.()
+  unsubscribe = null
+})
+</script>
+
+<template>
+  <div v-if="errors.length" class="global-error-stack" role="alert" aria-live="assertive">
+    <el-alert
+      v-for="item in errors"
+      :key="item.id"
+      :title="item.message"
+      type="error"
+      show-icon
+      :closable="true"
+      @close="dismissGlobalError(item.id)"
+    />
+  </div>
+</template>