Make an embeddable version of front-end (nitro.js)

**Is your feature request related to a problem? Please describe.**

The front-end currently expects all of `index.html` to be served from the backend, and requires a `/nitro` WS endpoint to be exposed by the server. This is avoidable.

**Describe the solution you'd like**

An embeddable version of the front-end that's purely data-in, data-out could make it easier to integrate with legacy software. The legacy software would have to include a bit of JS to wire up nitro.js to the backend.