Add feature flag to disable the TUI

[LucasPickering]

Description

Describe the change. If there is an associated issue, please include the issue link (e.g. "Closes #xxx"). For UI changes, please also include screenshots.

I don't think any users would ever want to do this, but it's helpful in dev when iterating and only the CLI is needed. Should speed up compilation a bit.

Known Risks

What issues could potentially go wrong with this change? Is it a breaking change? What have you done to mitigate any potential risks?

A user could be confused by this. Mitigated by having it enabled by default, and not documenting it so I doubt anyone will come across it other than in the Cargo.toml.

QA

How did you test this?

manually

Checklist

• Have you read CONTRIBUTING.md already?
• Did you update CHANGELOG.md?
  • Only user-facing changes belong in the changelog. Internal changes such as refactors should only be included if they'll impact users, e.g. via performance improvement.
• Did you remove all TODOs?
  • If there are unresolved issues, please open a follow-on issue and link to it in a comment so future work can be tracked