Shared personal authentication

Merchant and operator interfaces use the same native sign-in challenge contract. AuthenticationChallenge.tsx collects a second factor without storing a pending challenge as a session. Account response types live in shared/api/account-types.ts; all interface text lives in shared/i18n/security-i18n.ts.

The server owns password checks, expiry, attempt limits, replay prevention and current membership admission. Personal security is operational identity data and is never cloned, restored or published through a shop sandbox.