perf(website): size the hero poster to the box it paints in (#1062)

Closes #876

### What changed

`website/site/public/media/hero-poster.webp` is encoded at **1440×810**
instead of 1920×1080, and `scripts/build-media.mjs` resizes to 1440 so
the next `pnpm run media` keeps the size instead of restoring 1920.

1440 covers the box the film actually paints in: `Hero.astro` caps it at
`min(1180px, 80vw, film-h * 16/9)`, so at most 1180×664 CSS px — 1440 is
**1.22×** that, enough for a 1.25× display, and no `devicePixelRatio`
branch is needed. The 1359×764 in #876 was the layout before that cap
landed in #899, two days after the audit; the issue's numbers describe a
box the site no longer has.

### Before / after

| | bytes | dimensions |
|---|---|---|
| before | 42,926 | 1920 × 1080 |
| after | **30,878** | 1440 × 810 |

**−12,048 B, −28.1%.** Both below the issue's ≤ 45 KB bound.

Note the source file was already smaller than the 73,916 B the
2026-10-17 audit measured — it was re-encoded during the #858 rebrand —
so the absolute saving is smaller than the audit's 36.9 KiB estimate,
while the resolution waste it reported is removed.

### Visual check

SSIM of the new file against the old one, both scaled with lanczos to
the comparison size:

| comparison size | SSIM |
|---|---|
| 1180 × 664 (the film's CSS cap today) | 0.9968 |
| 1359 × 764 (the layout #876 measured) | 0.9964 |
| 2718 × 1528 (2× device pixels) | 0.9932 |

### `og:image` was never this file

The issue's second consumer no longer exists: `Base.astro` defaults
`ogImage` to `/media/social-card.png`, generated at 1200×630 by
`build-social-card.mjs` (#884). Nothing overrides it — `grep -rn
"ogImage=" website/site/src` returns only the default and the prop
declaration. So the "must stay 1920×1080" constraint does not apply, and
no second 1920 file is kept.

Built output confirms both:

```
dist/media/hero-poster.webp   30,878 B   1440,810
poster="/media/hero-poster.webp"
property="og:image" content="https://nostream.dev/media/social-card.png"
```

### Verification

- `pnpm build` in `website/site` succeeds (prebuild's brand + hero-NIP
checks included); the asset in `dist/media/` is the new one at 30,878 B.
- CLS: the film keeps `aspect-ratio: 16/9` on the `<video>`, unchanged
by this PR, so the poster's intrinsic size does not enter layout.

### Out of scope

`admin-poster.webp` and `peering-poster.webp` are encoded by the same
script at 1920 and have the same geometry mismatch. They are not
measured by this issue's audit and are left alone; the fix is one
`resize()` call each when someone measures them.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

<!-- coderabbit-ai-generated:v1 -->

## Summary by CodeRabbit

* **Performance**
* Reduced the landing page hero image width from 1920 to 1440 pixels,
lowering the image’s download size while preserving visual quality.
  * Social preview imagery remains unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: cameri

.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/public/media/hero-poster.webp

@@ -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})`)