fix: 切换实例按钮无效 & 磁盘分区内容溢出

- Select 下拉切换主机时同步更新 latest/serverInfo（添加 selectedHost 依赖）
- 磁盘分区 used_gb/usage_percent 格式化 .toFixed(1)，容器加宽 + whitespace-nowrap 防溢出

frontend/src/pages/HostMonitor.tsx

@@ -100,13 +100,14 @@ export function HostMonitor() {
         const m = urlHost ? list.find((h: any) => h.hostname === urlHost) : null;
         setSelectedHost(m?.hostname || list[0]?.hostname || '');
       }
-      const cur = list.find((h: any) => h.hostname === (urlHost || selectedHost));
+      // 切换主机时同步更新 latest / serverInfo
+      const cur = list.find((h: any) => h.hostname === (selectedHost || urlHost));
       if (cur) {
         setLatest(cur);
         setServerInfo(cur);
       }
     }).catch(() => {}).finally(() => setLoading(false));
-  }, []);
+  }, [selectedHost]);
 
   // 加载历史数据
   useEffect(() => {
@@ -392,8 +393,8 @@ export function HostMonitor() {
                         <div className="flex-1 h-1.5 bg-slate-700 rounded-full overflow-hidden">
                           <div className="h-full rounded-full bg-blue-500" style={{ width: `${Math.min(p.usage_percent || p.used_percent || 0, 100)}%` }} />
                         </div>
-                        <span className="text-[var(--dg-text-primary)] w-20 text-right tabular-nums">{p.used_gb?.toFixed(1) || p.used || '?'}/{p.total_gb?.toFixed(0) || p.total || '?'}GB</span>
-                        <span className="text-blue-400 w-12 text-right tabular-nums">{p.usage_percent || p.used_percent || 0}%</span>
+                        <span className="text-[var(--dg-text-primary)] w-28 text-right tabular-nums whitespace-nowrap">{(p.used_gb ?? 0).toFixed(1)}/{(p.total_gb ?? 0).toFixed(0)}GB</span>
+                        <span className="text-blue-400 w-14 text-right tabular-nums whitespace-nowrap">{(p.usage_percent ?? p.used_percent ?? 0).toFixed(1)}%</span>
                       </div>
                     ))}
                   </div>