The art manifest referenced /art/*.png files that never existed; the SPA
fallback returned index.html for them, so the library thumbnails rendered
as broken images. Generate 5 atmospheric B&W placeholder scenes
(tools/gen-art-placeholders.mjs, npm run gen:art) into public/art/.
tests/tools/art-assets.test.js ties every manifest entry to a real PNG.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
