Convert 26 PNG/JPG assets to WebP: max 1600 px, q80, diagrams lossless.
src/assets shrinks from 8.3 MB to 4.2 MB.
Use me.jpg for og:image and twitter:image; some crawlers skip WebP.
Add scripts/check-assets.mjs to lint, so a wrong asset path fails lint,
not only at runtime.
