The terminal's new-forge screen offers the same three sizes, and the chosen size reaches the plan #5681

Split from #2996: the terminal half. The reviewer of pull #5676 asked for a behaviour test here, not a grep of the source.

Done:

• [ ] src/tui/screens/newforge.ts offers small, medium and large, with small chosen. Choosing small records no size, as on the web form; medium or large is recorded.
• [ ] A new test/tui-newforge-size.test.ts drives the screen the way the other terminal screen tests do: it opens it, moves to the size, picks medium and submits. It asserts the plan carries size: "medium", and that submitting with small carries no size. It fails without the change.
• [ ] The existing terminal tests for the screen pass unchanged.

Files: src/tui/screens/newforge.ts, test/tui-newforge-size.test.ts. Risk: keep the screen's keys as they are; only add the size row.