Moves what the guest created to the signed-in account: its listings, likes and comments.

Bootstrap/CSRF of this browser required; the page does it right after sign-in. Only what the guest still owns moves, in one transaction, and anything that collides with what the account already has stays with the guest. What the guest bought moves too. An old unsigned token that owns nothing here is refused. Repeating it is harmless (moves zero). Returns: { ok, claimed }