perf(website): size the hero poster to the box it paints in

.changeset/website-hero-poster-resolution.md

@@ -0,0 +1,19 @@
+---
+"nostream-website": patch
+---
+
+perf(website): size the hero poster to the box it paints in
+
+`hero-poster.webp` was encoded at 1920×1080 for a box `Hero.astro` caps at
+1180×664 CSS px (`min(1180px, 80vw, film-h * 16/9)`), which Lighthouse's
+`image-delivery-insight` reported as 36.9 KiB of waste on
+`ema.tnsor.network` ([#876](https://github.com/cameri/nostream/issues/876)).
+The poster now encodes at 1440×810 — 1.22× that box, enough for a 1.25×
+display, so it needs no `devicePixelRatio` branch — and drops from 42,926 B
+to 30,878 B, a 28% reduction. SSIM against the old file at the display size
+is 0.9968 at 1180×664 and 0.9964 at the larger 1359×764 layout #876 measured.
+
+The social preview is unaffected: `og:image` defaults to the generated
+`social-card.png` at 1200×630 in `build-social-card.mjs`, and the hero poster
+has no second consumer. `scripts/build-media.mjs` resizes to 1440 so the next
+`pnpm run media` keeps the size instead of restoring 1920.

website/site/scripts/build-media.mjs

@@ -35,9 +35,15 @@ if (existsSync(planet)) {
   console.warn('skip planet: run `pnpm run assets` in website/hero-animation first')
 }
 
+// Page-sized, not film-sized: Hero.astro caps the film at `min(1180px, 80vw,
+// film-h * 16/9)`, so the poster paints in at most 1180x664 CSS px and 1440x810
+// is 1.22x that box - enough for a 1.25x display, and no devicePixelRatio
+// branch. (#876 measured 1359x764, the layout before that cap landed in #899.)
+// The social preview is `social-card.png`, generated at 1200x630 by
+// build-social-card.mjs, and never this file.
 const landingPoster = path.join(LANDING, 'frames', LANDING_POSTER)
 if (existsSync(landingPoster)) {
-  await sharp(landingPoster).resize(1920).webp({ quality: 90 }).toFile(path.join(MEDIA, 'hero-poster.webp'))
+  await sharp(landingPoster).resize(1440).webp({ quality: 90 }).toFile(path.join(MEDIA, 'hero-poster.webp'))
   console.log('hero poster → public/media')
 } else {
   console.warn(`skip hero poster: run \`pnpm run frames\` in website/hero-animation (need frames/${LANDING_POSTER})`)