feat: add hero image processing and API for serving variants

- Introduced a new script `prepare-hero-variants.mjs` to generate responsive hero image variants in WebP format.
- Added a fallback image `hero_fallback.png` for each variant.
- Created an API endpoint `hero-images.get.js` to retrieve available hero image variants and their fallback images.
- Implemented directory and file checks to ensure the existence of required images before serving.

assets/images/hero-originals/README.md

@@ -0,0 +1,20 @@
+# Hero-Originalbilder
+
+Das Tool `npm run hero:prepare` unterstuetzt zwei Eingabeformate:
+
+1. Unterordner pro Variante (empfohlen)
+
+- `assets/images/hero-originals/<variante>/hero.png`
+
+2. Flache Ablage von PNGs
+
+- `public/images/hero-originals/hero1.png`
+- `public/images/hero-originals/hero2.png`
+
+Ausgabe je Variante in:
+
+- `public/images/hero/<variante>/hero_960.webp`
+- `public/images/hero/<variante>/hero_1600.webp`
+- `public/images/hero/<variante>/hero_fallback.png`
+
+Die Startseite (`components/Hero.vue`) waehlt danach automatisch zufaellig eine vorhandene Variante aus.