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.

server/api/hero-images.get.js

@@ -0,0 +1,87 @@
+import { promises as fs } from 'fs'
+import path from 'path'
+
+const HERO_ROOT_CANDIDATES = [
+  path.join(process.cwd(), 'public', 'images', 'hero'),
+  path.join(process.cwd(), '.output', 'public', 'images', 'hero'),
+  path.join(process.cwd(), '..', 'public', 'images', 'hero'),
+  path.join(process.cwd(), '..', '.output', 'public', 'images', 'hero')
+]
+
+const FALLBACK_FILE_CANDIDATES = [
+  'hero_fallback.webp',
+  'hero_fallback.jpg',
+  'hero_fallback.jpeg',
+  'hero_fallback.png'
+]
+
+async function findExistingDir(paths) {
+  for (const candidate of paths) {
+    try {
+      const stats = await fs.stat(candidate)
+      if (stats.isDirectory()) return candidate
+    } catch {
+      // ignore
+    }
+  }
+  return null
+}
+
+async function fileExists(filePath) {
+  try {
+    const stats = await fs.stat(filePath)
+    return stats.isFile()
+  } catch {
+    return false
+  }
+}
+
+async function listHeroVariants(heroRoot) {
+  const dirEntries = await fs.readdir(heroRoot, { withFileTypes: true })
+  const variants = []
+
+  for (const entry of dirEntries) {
+    if (!entry.isDirectory()) continue
+
+    const key = entry.name
+    const variantDir = path.join(heroRoot, key)
+    const mobileFile = 'hero_960.webp'
+    const desktopFile = 'hero_1600.webp'
+
+    const mobilePath = path.join(variantDir, mobileFile)
+    const desktopPath = path.join(variantDir, desktopFile)
+
+    if (!(await fileExists(mobilePath)) || !(await fileExists(desktopPath))) {
+      continue
+    }
+
+    let fallbackFile = desktopFile
+    for (const candidate of FALLBACK_FILE_CANDIDATES) {
+      if (await fileExists(path.join(variantDir, candidate))) {
+        fallbackFile = candidate
+        break
+      }
+    }
+
+    variants.push({
+      key,
+      mobileWebp: `/images/hero/${key}/${mobileFile}`,
+      desktopWebp: `/images/hero/${key}/${desktopFile}`,
+      fallback: `/images/hero/${key}/${fallbackFile}`
+    })
+  }
+
+  return variants.sort((a, b) => a.key.localeCompare(b.key, 'de'))
+}
+
+export default defineEventHandler(async (event) => {
+  const heroRoot = await findExistingDir(HERO_ROOT_CANDIDATES)
+  if (!heroRoot) {
+    return { variants: [] }
+  }
+
+  const variants = await listHeroVariants(heroRoot)
+  setHeader(event, 'Cache-Control', 'public, max-age=300, stale-while-revalidate=600')
+
+  return { variants }
+})