chore: remove ui subfolder

[ellie]

This is still in development, but rather than clutter the commit history and issues with an unreleased project I've split the UI into its own repo.

Once ready for release, I'll either merge the ui code back in, or just make the repo public.

Checks

• [ ] I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle
• [ ] I have checked that there are no existing pull requests for the same thing