Add default field to profiles

[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.

For the CLI, the default profile will be used when --profile isn't passed. For the TUI, we'll just preselect this field on first startup. Only one profile can be marked as default, and that's enforced during deserialization.

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?

Default enforcement could be broken. Mitigated with a test. CLI is untested.

QA

How did you test this?

Unit tests, tested CLI 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