refactor: change agent cloud data permission to cloud variables only

- Rename permission from '查看云端数据' to '查看云端变量'
- Remove cloud constants API and page for agents
- Add cloud variable records API for agents (stream type)
- Add agent cloud-variables page with full variable details
- Add agent cloud-variable records page (read-only)
- Update navigation from cloud-data to cloud-variables
- Update i18n keys: cloudData -> cloudVariables
- Backend returns var_type, default_value, max_records etc.
- Records page supports dynamic columns and date filtering

frontend/src/router/routes.ts

@@ -451,10 +451,16 @@ const routes: RouteRecordRaw[] = [
         meta: { title: '财务管理 - 代理商后台' },
       },
       {
-        path: 'cloud-data',
-        name: 'AgentCloudData',
-        component: () => import('@/pages/agent/cloud-data/index.vue'),
-        meta: { title: '云端数据 - 代理商后台' },
+        path: 'cloud-variables',
+        name: 'AgentCloudVariables',
+        component: () => import('@/pages/agent/cloud-variables/index.vue'),
+        meta: { title: '云端变量 - 代理商后台' },
+      },
+      {
+        path: 'cloud-variables/:id/records',
+        name: 'AgentCloudVariableRecords',
+        component: () => import('@/pages/agent/cloud-variables/[id]/records.vue'),
+        meta: { title: '变量记录 - 代理商后台' },
       },
       {
         path: 'profile',