feat: add admin UI frontend and complete backend APIs

- Add Vue 3 frontend with Element Plus
- Implement login, dashboard, tenant management
- Add app configuration, logs viewer, stats pages
- Add user management for admins
- Update Drone CI to build and deploy frontend
- Frontend ports: 3001 (test), 4001 (prod)

frontend/src/main.js

@@ -0,0 +1,23 @@
+import { createApp } from 'vue'
+import { createPinia } from 'pinia'
+import ElementPlus from 'element-plus'
+import zhCn from 'element-plus/dist/locale/zh-cn.mjs'
+import * as ElementPlusIconsVue from '@element-plus/icons-vue'
+import 'element-plus/dist/index.css'
+
+import App from './App.vue'
+import router from './router'
+import './assets/styles/main.scss'
+
+const app = createApp(App)
+
+// 注册所有图标
+for (const [key, component] of Object.entries(ElementPlusIconsVue)) {
+  app.component(key, component)
+}
+
+app.use(createPinia())
+app.use(router)
+app.use(ElementPlus, { locale: zhCn })
+
+app.mount('#app')