chore: achieve 100/100 pagespeed and html validation

- fix html validation errors in blog mdx (empty headings)
- fix backstopjs esm compatibility and missing reference images
- optimize product data structure and next.config.mjs
- finalize accessibility and seo improvements

next.config.mjs

@@ -348,6 +348,10 @@ const nextConfig = {
     }
 
     return [
+      {
+        source: '/de/produkte',
+        destination: '/de/products',
+      },
       {
         source: '/cms/:path*',
         destination: `${directusUrl}/:path*`,