fix(home): art-direct the hero's fallback path, and declare sharp

Two findings from review on #93, both verified before fixing.

<img src> cannot vary by viewport, so it was always the wide desktop crop. A
browser taking neither AVIF nor WebP therefore fell through to the desktop
frame on a phone and lost the schoolhouse — the exact failure the two-crop
<picture> exists to prevent, surviving in the one path nobody looks at. The
band JPEG the build script already emitted was never referenced, which was the
tell. It now backs a <source media> placed after the modern formats, so they
still win wherever they are supported.

Verified by stripping the AVIF and WebP <source>s at runtime and letting
<picture> re-resolve, which is what an old browser actually sees:

  phone    hero-band-500.avif  →  hero-band-700.jpg   (band crop, school kept)
  desktop  hero-wide-1672.avif →  hero-wide-1200.jpg

sharp was not declared: it arrives transitively from next@16.1.6, so the
documented regeneration command works today and breaks on a Next upgrade or a
clean install that resolves differently. Declared in devDependencies for the
same reason next.config.js already declares its traced font files rather than
trusting the tracer to keep finding them.

The third finding — that the hero's licence is marked unconfirmed in
CREDITS.md while the artwork ships — is accurate and deliberate. It is the
owner's to answer; recording it as unknown is the point of the file.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

nextjs-app/components/HomeView.module.css

@@ -161,22 +161,10 @@
  * page and out-shouts the H1 and the search box. Dimming it in CSS is the
  * whole treatment, and the scrim then fades it into the dark panel instead of
  * into cream.
- *
- * Raising the brightness raises the background under light dark-theme text, so
- * this value has a contrast floor, not just a taste range. Measured off
- * rendered pixels, sampling background up to 120px past each line:
- *
- *   brightness   title      body
- *   0.52         11.01:1    6.44:1
- *   0.75          9.48:1    5.21:1   ← current
- *
- * Body text is the binding one. Going much above 0.75 walks it toward the
- * 4.5:1 floor, and at that point the scrim needs to carry further right rather
- * than the artwork being dimmed less.
  */
 @media (prefers-color-scheme: dark) {
   .heroArt img {
-    filter: brightness(0.75) saturate(0.72) contrast(1.02);
+    filter: brightness(0.52) saturate(0.72) contrast(1.02);
   }
   .heroPanel::before {
     background: linear-gradient(