refactor(frontend): P0 API 层收口与 http 统一

- 统一 http.ts baseURL：get/post/download/upload 均走 resolveApiUrl
- agentUtils 下沉至 lib/agent-utils，API 层移除对 components 的依赖
- 扩展 metricsApi/snmpApi/prometheusProxyApi/zabbixProxyApi
- 扩展 pipelineApi/credentialApi 覆盖流水线与凭证场景
- 14 个组件/页面禁止直连 http，统一走 @/services/api

frontend/src/components/monitor/dashboard/GrafanaDashboard.tsx

@@ -39,8 +39,7 @@ import { dashboardApi, promDatasourceApi, zabbixDatasourceApi, agentApi, serverA
 import { GrafanaPanelContent } from './GrafanaPanelContent';
 import { ZabbixDashboard } from './ZabbixDashboard';
 import { PrometheusDashboard } from './PrometheusDashboard';
-import { http } from '@/services/http';
-import { asList } from '@/components/agent/agentUtils';
+import { asList } from '@/lib/agent-utils';
 import { AgentWizard } from '@/components/agent/AgentWizard';
 import { Checkbox } from '@/components/ui/checkbox';
 import { useConfirm } from '@/components/ui/confirm-dialog';
@@ -94,7 +93,7 @@ const AgentTab: React.FC = () => {
     finally { setBatchUninstalling(false); }
   };
   const totalPages = Math.max(1, Math.ceil(hosts.length / pageSize));
-  const fetch = async () => { try { setHosts(asList(await http.get('/agent/latest'))); } catch {} finally { setLoading(false); } };
+  const fetch = async () => { try { setHosts(asList(await agentApi.getLatest())); } catch {} finally { setLoading(false); } };
   useEffect(() => { fetch(); const t = setInterval(fetch, 30000); return () => clearInterval(t); }, []);
   const toggle = (f: SortField) => { if (sf === f) setSd(d => d === 'asc' ? 'desc' : 'asc'); else { setSf(f); setSd('desc'); } };
   const sorted = [...hosts].sort((a, b) => {