feat: 添加功能开关机制

- 添加环境变量配置 VITE_FEATURE_DUO_PRACTICE 等
- env.ts 新增 isFeatureEnabled 方法
- 菜单根据功能开关动态显示/隐藏
- 路由守卫拦截未启用功能的直接访问
- 开发环境默认开启双人对练，生产环境默认关闭

frontend/src/router/index.ts

@@ -137,25 +137,25 @@ const routes: RouteRecordRaw[] = [
         path: 'duo-practice',
         name: 'DuoPractice',
         component: () => import('@/views/trainee/duo-practice.vue'),
-        meta: { title: '双人对练', icon: 'Connection' }
+        meta: { title: '双人对练', icon: 'Connection', feature: 'duo-practice' }
       },
       {
         path: 'duo-practice/room/:code',
         name: 'DuoPracticeRoom',
         component: () => import('@/views/trainee/duo-practice-room.vue'),
-        meta: { title: '对练房间', hidden: true }
+        meta: { title: '对练房间', hidden: true, feature: 'duo-practice' }
       },
       {
         path: 'duo-practice/join/:code',
         name: 'DuoPracticeJoin',
         component: () => import('@/views/trainee/duo-practice-room.vue'),
-        meta: { title: '加入对练', hidden: true }
+        meta: { title: '加入对练', hidden: true, feature: 'duo-practice' }
       },
       {
         path: 'duo-practice/report/:id',
         name: 'DuoPracticeReport',
         component: () => import('@/views/trainee/duo-practice-report.vue'),
-        meta: { title: '对练报告', hidden: true }
+        meta: { title: '对练报告', hidden: true, feature: 'duo-practice' }
       }
     ]
   },