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

backstop.config.cjs

@@ -1,4 +1,6 @@
-/* eslint-disable */
+const BASE_URL = process.env.TEST_URL || 'http://localhost:3000';
+const REFERENCE_URL = process.env.REFERENCE_URL || 'https://klz-cables.com';
+
 module.exports = {
   id: 'klz-cables',
   viewports: [
@@ -18,13 +20,13 @@ module.exports = {
       height: 900,
     },
   ],
-  onBeforeScript: 'puppet/onBefore.js',
-  onReadyScript: 'puppet/onReady.js',
+  onBeforeScript: 'puppet/onBefore.cjs',
+  onReadyScript: 'puppet/onReady.cjs',
   scenarios: [
     {
       label: 'Homepage',
-      url: `${process.env.TEST_URL || 'http://host.docker.internal:3000'}/`,
-      referenceUrl: '',
+      url: `${BASE_URL}/`,
+      referenceUrl: `${REFERENCE_URL}/`,
       readyEvent: '',
       readySelector: '',
       delay: 500,
@@ -41,7 +43,8 @@ module.exports = {
     },
     {
       label: '404 Error Page',
-      url: `${process.env.TEST_URL || 'http://host.docker.internal:3000'}/this-page-does-not-exist`,
+      url: `${BASE_URL}/this-page-does-not-exist`,
+      referenceUrl: `${REFERENCE_URL}/this-page-does-not-exist`,
       delay: 500,
       misMatchThreshold: 0.1,
     },
@@ -53,7 +56,7 @@ module.exports = {
     html_report: 'backstop_data/html_report',
     ci_report: 'backstop_data/ci_report',
   },
-  report: process.env.CI ? ['CI'] : ['browser'],
+  report: process.env.CI ? ['CI', 'json'] : ['browser'],
   engine: 'puppeteer',
   engineOptions: {
     args: ['--no-sandbox', '--disable-setuid-sandbox'],