background-image data URLs on divs can fail silently in StPageFlip's
shadow DOM context, producing white pages. Switch to <img> elements
with explicit pixel dimensions and object-fit:cover for reliable
rendering. Add white background to .stf__item as a safety net.
Applies to both folk-pubs-flipbook and folk-book-reader.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>