BookingSubPageLayout

import BookingSubPageLayout from "@dertour/ui/components/common/BookingSubPageLayout/BookingSubPageLayout"

Shared chrome for the booking-detail sub-pages (billing, documents, passenger info): a "Back to Booking" link and a two-column body with the expert-contact card on the left and the page content on the right (stacks below md). Uses the same `SectionWrapper` (container width + top spacing) as the booking-detail page so the back link and content line up consistently across all of them.