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

data/blog/en/securing-the-future-with-h1z2z2-k-our-solar-cable-for-intersolar-2025.mdx

@@ -94,7 +94,6 @@ One major advantage of the H1Z2Z2-K is its suitability for direct burial – wit
 
 **Important:** For projects spanning several hundred meters, a voltage drop calculation is worthwhile – 6mm² isn’t always the best fit by default.
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 ## FAQ: The most frequently asked questions about H1Z2Z2-K solar cables
 **What does H1Z2Z2-K mean?**<br />This designation refers to a cable type with specific insulation materials and properties according to EN 50618, suitable for DC voltage up to 1500 V.
 **Is the cable approved for underground installation?**<br />Yes, including direct burial without additional protective conduits.