Question bank import hangs on large files

The Excel question bank import appears to hang at the end of the import when the imported question set is large: the import dialog stays blocked with the uploaded file shown and seems never to finish. The imported questions are ultimately added to the subject successfully, so the failure is limited to the completion feedback of the import dialog rather than the import itself. The dialog should clearly indicate completion (or fail with a clear error) instead of remaining stuck after a large import.