fix: defer revoking group file download URL

web/src/features/groups/GroupFilesModal.tsx

@@ -128,7 +128,8 @@ export function GroupFilesModal({ topicId, isAdmin, canUpload, onClose }: GroupF
         link.href = url;
         link.download = f.file_name;
         link.click();
-        URL.revokeObjectURL(url);
+        // Keep the object URL alive long enough for the browser to start the download.
+        window.setTimeout(() => URL.revokeObjectURL(url), 1000);
       })
       .catch(() => setDownloadError('文件下载失败，可能已过期'));