Fix blog feature image: use heroImage from frontmatter instead of hardcode

src/pages/blog/[slug].astro

@@ -138,11 +138,7 @@ const faqPageJsonLd = faqPairs.length > 0
     </header>
 
     <div class="blog-feature-image">
-      {['ai-for-sme-start-with-pain', 'marketing-automation-reduce-work', 'sme-website-next-action'].includes(post.id) ? (
-        <img src={`/images/blog-${post.id === 'ai-for-sme-start-with-pain' ? 'ai' : post.id === 'marketing-automation-reduce-work' ? 'automation' : 'website'}.jpg`} alt={post.data.title} />
-      ) : (
-        <img src="/images/blog-marketing.jpg" alt={post.data.title} />
-      )}
+      <img src={post.data.heroImage || '/images/blog-placeholder.svg'} alt={post.data.title} />
     </div>
 
     <div class="blog-article-shell">