[v4] Canvas theme

A great theme would be a canvas in which you can link the Diffuse elements.

Resources:

• https://www.npmjs.com/package/@folkjs/canvas
• https://www.npmjs.com/package/@folkjs/propagators
• https://github.com/folk-js/folkjs/blob/main/website/demos/propagators/web-applets.html
• https://tldraw.dev/examples/custom-embed