feat: add balance display, user columns, device mgmt and sessions for agent

- Backend: add balance to agent stats API
- Backend: return full user details (balance, expiry, online_status, device_count, application) in agent users API
- Backend: export admin device/session handlers, add agent routes for devices and sessions
- Backend: add card type permission check and balance deduction for agent user creation
- Backend: extract checkAgentUserPermission helper, add user edit API
- Frontend: add balance card to agent dashboard
- Frontend: add application, online_status, device_count, balance, expiry_at columns to agent user table
- Frontend: add agent devices page (reuse admin data-table component)
- Frontend: add agent sessions page (reuse admin data-table component)
- Frontend: add devices and sessions to agent navigation and routes
- Add i18n keys for agent devices, sessions, user columns, dashboard balance

frontend/src/router/routes.ts

@@ -462,6 +462,18 @@ const routes: RouteRecordRaw[] = [
         component: () => import('@/pages/agent/finance/index.vue'),
         meta: { title: '财务管理 - 代理商后台' },
       },
+      {
+        path: 'devices',
+        name: 'AgentDevices',
+        component: () => import('@/pages/agent/devices/index.vue'),
+        meta: { title: '设备管理 - 代理商后台' },
+      },
+      {
+        path: 'sessions',
+        name: 'AgentSessions',
+        component: () => import('@/pages/agent/sessions/index.vue'),
+        meta: { title: '在线实例 - 代理商后台' },
+      },
       {
         path: 'cloud-variables',
         name: 'AgentCloudVariables',