Invites, part 2b: the terminal's create-account screen asks for an invite when signup = invite #2418

Part of #2336.

Done looks like:

• [ ] When signup = invite, the terminal's create-account screen (it already goes through signupAllowed, #2398) asks for the invite token and redeems it the same way the web form does.
• [ ] test/tui-can.test.ts ("what I can do in the terminal") is updated only where the screen's list deliberately changes, and the pull says which line changed and why.
• [ ] Test: with a good token the account is made; with a bad one there is one plain refusal.