Cleanup of failed download may misbehave

Sometimes if a download fails, the subsequent cleanup of the project directory may fail as well: https://github.com/MerginMaps/qgis-mergin-plugin/issues/442#issuecomment-1212909680
While it looks like some other app is using the downloaded files, it is possible that we are doing the cleanup too early - e.g. in download_project_is_running() we do the cleanup while some other futures may be still active: we should wait until all jobs are done before doing any cleanup.

从 download_project_is_running() 开始，跟踪项目目录清理与其他活动 future 的关系。使用链接的 Mergin Maps issue 了解所报告的失败场景。完成的标准是：清理会等待所有下载任务完成，并且在下载失败时不再失败。