fix: 移除getHistory中setLoading(true)避免切换主机时下拉框闪烁消失

frontend/src/pages/HostMonitor.tsx

@@ -153,7 +153,6 @@ export function HostMonitor() {
 
   useEffect(() => {
     if (!selectedHost) return;
-    setLoading(true);
     agentApi.getHistory(selectedHost, rangeHours)
       .then((data: any) => {
         const list = Array.isArray(data) ? data : (data?.data || data?.list || []);