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/layouts/agent.vue

@@ -93,8 +93,8 @@ const navMain = computed(() => {
 
   if (user.can_view_cloud_data) {
     groups[0].items.push({
-      title: '云端数据',
-      url: '/agent/cloud-data',
+      title: t('nav.cloudVariables'),
+      url: '/agent/cloud-variables',
       icon: Cloud,
     })
   }
@@ -122,7 +122,7 @@ const breadcrumbs = computed(() => {
     cards: t('nav.cards'),
     users: t('nav.users'),
     finance: t('nav.finance'),
-    'cloud-data': '云端数据',
+    'cloud-variables': t('nav.cloudVariables'),
     profile: t('nav.profile'),
     create: t('nav.create'),
   }