Fixed offer Portal preview sticky CTA behavior (#28188)

ref
https://linear.app/ghost/issue/DES-1272/footer-scroll-bug-in-portal-settings-preview

Fixed the offer Portal preview so the sticky CTA stays pinned to the
bottom while scrolling, while preserving the intended scaled preview
size and bottom spacing.

This uses `zoom` instead of `transform: scale()` because `transform`
only changes the visual rendering, while sticky positioning is still
calculated from the unscaled layout box.