RayXpress Multilingual Fix

Fixed:
1. Public language selection now persists in the session across page navigation and reloads.
2. Public country detection no longer gets incorrectly overridden by the browser's locale region; the server's country header is used first, with browser language as fallback.
3. Translation API requests now use strict JSON-schema output with an ordered translation array, making the Responses API response parsing reliable.
4. Master-selected shipment language now takes priority when an explicit language is supplied to shipment email delivery. This is required for guest shipments created before registration.
5. Translation failures now surface a small diagnostic message in the public page instead of silently leaving the page in English.

No database migration is required.
