I love it! I will absolutely be trying this out and following along.
For a long time I've used draw.io for making diagrams, and found it clever that you can effectively save your document as an iframe blob of editor + file (though in their case, the editor is not stored in the file, a web connection is required to load the editor codebase. I've wished that could be the primary use case and this is basically what I'd imagined!
Thanks! As a avid user of draw.io, I considered this model, but ultimately I thought that it's best to start with a single local file. Maybe later when people ask for enterprise-type features it'd make sense to host it. It's definitely needed to do stuff like SSO.