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.

從 download_project_is_running() 開始，追蹤專案目錄清理與其他作用中的 future 之間的關係。使用連結的 Mergin Maps issue 了解所回報的失敗情境。完成的標準是：清理會等待所有下載工作完成，並且在下載失敗時不再失敗。