Add blog post: How to Capture Dark Mode Screenshots Automatically

- New blog post covering darkMode parameter, CSS injection, hideSelectors, dual OG images
- Code examples in cURL, Node.js, and Python
- Blog index updated with new post
- Sitemap updated with new URL
- Tests already committed (474 passing)

public/blog.html

@@ -93,6 +93,13 @@ footer{border-top:1px solid var(--border);padding:48px 24px 32px;background:var(
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   <div class="blog-grid">
+    <article class="blog-card">
+      <div class="meta"><span>March 6, 2026</span><span>6 min read</span></div>
+      <h2><a href="/blog/dark-mode-screenshots">How to Capture Dark Mode Screenshots Automatically</a></h2>
+      <p>Dark mode isn't a trend anymore — it's the default. Learn how to capture both light and dark screenshots with a single API parameter, inject custom dark themes, and generate dual OG images for social media.</p>
+      <a href="/blog/dark-mode-screenshots" class="read-more">Read article →</a>
+    </article>
+
     <article class="blog-card">
       <div class="meta"><span>March 3, 2026</span><span>7 min read</span></div>
       <h2><a href="/blog/automating-og-images">Automating OG Image Generation with Screenshot APIs</a></h2>