Providing a web app with edit access to a local directory is really needed for this to be usable. Without that you're constantly managing downloaded files and manually replacing things. I do think this is a case where the File System Access API shines.
> Providing a web app with edit access to a local directory is really needed for this to be usable.
"This" what? sha256sum doesn't need read-write access for even one file to be able to compute a hash, let alone a whole directory. You're ignoring most of my comment, focusing on like 20%, and in so doing, missing (and/or deliberately misframing) 100% of the point.