fix(tui): keep custom question answer per-tab

[luispl77]

Fixes #8199

Summary

• Commit "Type your own answer" value before switching question tabs / going to Confirm.
• When re-opening the custom input, move cursor to end of existing text.

Test Plan

• Started bun dev, triggered Question tool with multiple questions.
• Typed a custom answer on Question A, clicked Question B tab with mouse, then Confirm.
• Verified the custom answer stays on the correct question and cursor placement is sane.

Evidence

See before/after videos on #8199.