Replicate the Autotrader pattern: pull the page-back control out of the
sticky section-nav bar and place a standalone "← Back" text link above the
header card, on the page background, where it scrolls away with the page.
It keeps the context-aware behaviour (router.back → /search fallback); the
secondary view gains the same fallback for parity (it previously used a bare
router.back() that dead-ends on deep links).
The pinned slot the Back button vacated now holds a "↑ Top" control that
smooth-scrolls to the top, so the sticky bar keeps a useful affordance.
Applies to both primary and secondary detail views.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
