Reads thumbnailAspect from the backend and swaps between a 120×168
portrait tile and a 168×120 landscape tile so postcards and photos
don't get cropped into a portrait frame. Shows a page-count badge
top-right for multi-page PDFs, and a pulsing skeleton while the
async thumbnail job hasn't run yet. URL assembly goes through the
existing thumbnailUrl helper so cache-busting stays consistent
with DocumentThumbnail.
Refs #305
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
