This cross my mind too (as a why hasn't this been built?).

For now I think a few meh options:

You let the customer BYO encryption keys. You need tech savvy customers.

You offer to install on customers cloud. Also tech savvy.

For some solutions, a web front end that talks to Dropbox etc. may suffice. You just serve up static HTML.

Desktop app.

Web app with download/upload like Exaclidraw.