feat: 添加完整的前端管理系统 (VbenAdmin)

- 添加基于 VbenAdmin + Vue3 + Element Plus 的前端管理系统
- 包含完整的 UI 组件库和工具链
- 支持多应用架构 (web-ele, backend-mock, playground)
- 包含完整的开发规范和配置
- 修复 admin 目录的子模块问题，确保正确提交

admin/docs/src/demos/vben-api-component/cascader/index.vue

@@ -0,0 +1,100 @@
+<script lang="ts" setup>
+import { ApiComponent } from '@vben/common-ui';
+
+import { Cascader } from 'ant-design-vue';
+
+const treeData: Record<string, any> = [
+  {
+    label: '浙江',
+    value: 'zhejiang',
+    children: [
+      {
+        value: 'hangzhou',
+        label: '杭州',
+        children: [
+          {
+            value: 'xihu',
+            label: '西湖',
+          },
+          {
+            value: 'sudi',
+            label: '苏堤',
+          },
+        ],
+      },
+      {
+        value: 'jiaxing',
+        label: '嘉兴',
+        children: [
+          {
+            value: 'wuzhen',
+            label: '乌镇',
+          },
+          {
+            value: 'meihuazhou',
+            label: '梅花洲',
+          },
+        ],
+      },
+      {
+        value: 'zhoushan',
+        label: '舟山',
+        children: [
+          {
+            value: 'putuoshan',
+            label: '普陀山',
+          },
+          {
+            value: 'taohuadao',
+            label: '桃花岛',
+          },
+        ],
+      },
+    ],
+  },
+  {
+    label: '江苏',
+    value: 'jiangsu',
+    children: [
+      {
+        value: 'nanjing',
+        label: '南京',
+        children: [
+          {
+            value: 'zhonghuamen',
+            label: '中华门',
+          },
+          {
+            value: 'zijinshan',
+            label: '紫金山',
+          },
+          {
+            value: 'yuhuatai',
+            label: '雨花台',
+          },
+        ],
+      },
+    ],
+  },
+];
+/**
+ * 模拟请求接口
+ */
+function fetchApi(): Promise<Record<string, any>> {
+  return new Promise((resolve) => {
+    setTimeout(() => {
+      resolve(treeData);
+    }, 1000);
+  });
+}
+</script>
+<template>
+  <ApiComponent
+    :api="fetchApi"
+    :component="Cascader"
+    :immediate="false"
+    children-field="children"
+    loading-slot="suffixIcon"
+    visible-event="onDropdownVisibleChange"
+  />
+</template>