PayBox link of the authenticated wallet

Whether the caller's wallet is linked through Continue with PayBox, and whether that link can sign (status: linked, signing_key_missing, unlinked_by_paybox, unlinked_by_user, superseded or unlinked).

Asks PayBox once on every call, so a link the owner revoked IN PayBox reads here as unlinked_by_paybox — and from then on it cannot sign anything. Never returns a token or a key.