fix(ui): suppress update dialog during checking phase

Remove 'checking' from the render condition to prevent the dialog
from flashing briefly when auto-checking for updates on app startup.

tauri-app/src/components/UpdateDialog/UpdateDialog.tsx

@@ -38,7 +38,9 @@ export default function UpdateDialog() {
     return () => clearTimeout(timer);
   }, [check]);
 
-  if (!hasUpdate && !checking && !downloading && !installing && !error) {
+  // 只在有实质内容时显示弹窗：发现更新 / 正在下载 / 安装完成 / 出错
+  // checking 阶段不显示，避免一闪而过的白屏/loading
+  if (!hasUpdate && !downloading && !installing && !error) {
     return null;
   }