fix: 完善菜单和应用验证

- 侧边栏菜单增加「应用管理」和「企微应用」
- 租户订阅页面应用选择增加前端验证
- 后端增加 app_code 存在性验证

frontend/src/components/Layout.vue

@@ -13,7 +13,9 @@ const menuItems = computed(() => {
   const items = [
     { path: '/dashboard', title: '仪表盘', icon: 'Odometer' },
     { path: '/tenants', title: '租户管理', icon: 'OfficeBuilding' },
-    { path: '/app-config', title: '应用配置', icon: 'Setting' },
+    { path: '/apps', title: '应用管理', icon: 'Grid' },
+    { path: '/tenant-wechat-apps', title: '企微应用', icon: 'ChatDotRound' },
+    { path: '/app-config', title: '租户订阅', icon: 'Setting' },
     { path: '/stats', title: '统计分析', icon: 'TrendCharts' },
     { path: '/logs', title: '日志查看', icon: 'Document' }
   ]