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/packages/effects/layouts/src/widgets/index.ts

@@ -0,0 +1,11 @@
+export { default as Breadcrumb } from './breadcrumb.vue';
+export * from './check-updates';
+export { default as AuthenticationColorToggle } from './color-toggle.vue';
+export * from './global-search';
+export { default as LanguageToggle } from './language-toggle.vue';
+export { default as AuthenticationLayoutToggle } from './layout-toggle.vue';
+export * from './lock-screen';
+export * from './notification';
+export * from './preferences';
+export * from './theme-toggle';
+export * from './user-dropdown';