Rebuild the listing page around a full-bleed hero and add a photo viewer

[AndrewG828]

Stacked PR 13 of 15. Based on #122 — merge in order.
#111 → #112 → #113 → #114 → #115 → #116 → #117 → #118 → #119 → #120 → #121 → #122 → #123 → #124 → #125

The listing page becomes a photo-first layout: a 65%-height hero that the
details card overlaps with a rounded top edge, a swipeable pager with a photo
counter, and a tap-to-open full screen viewer with pinch, pan and double-tap
zoom.

Toolbar icons sit directly on the photo, so their color can't be fixed.
updateHeroIconContrast() samples the strip of the current image that actually
falls under the toolbar and picks black or white; a geometry observer flips it
back to black once the hero scrolls out from under the bar. Both transitions are
animated so a swipe between a dark and a light photo doesn't snap.

Details for reviewers:

• The viewer's paging is disabled while an image is zoomed in, otherwise the
  drag that pans a zoomed photo also flicks to the next one.
• Zoom offsets are clamped to the scaled image bounds, so you cannot pan a photo
  off screen and be left looking at a black rectangle.
• The page carries its own bottom inset. Pushed destinations don't inherit the
  tab root's clearance, so Similar Items sat underneath the floating tab bar.
• The custom back chevron replaces the system one, which takes the interactive
  swipe-back gesture with it — hence enableSwipeBack().

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_017R8mua9xepzF3mnRETtd6W

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