Accessibility: fix WCAG structural/landmark issues

- nav.php: add aria-label="Main navigation" to navbar <nav>
- article-footer.php: h4 -> h3 in related-card (h2->h4 skipped h3)
- 6 articles: add aria-label="Table of contents" to article-toc <nav>
- index.php: move trust-signals section inside <main> (was after </main>)
- web-scraping, price-monitoring, competitive-intelligence: add <main> wrapper
- index.php: inline #179e83 colour fixes (text on tinted backgrounds)

includes/article-footer.php

@@ -118,7 +118,7 @@ $display_articles = array_slice($filtered_articles, 0, 3);
         <?php foreach ($display_articles as $article): ?>
         <article class="related-card">
             <span class="category"><?php echo htmlspecialchars($article['category']); ?></span>
-            <h4><a href="/blog/articles/<?php echo htmlspecialchars($article['slug']); ?>.php"><?php echo htmlspecialchars($article['title']); ?></a></h4>
+            <h3><a href="/blog/articles/<?php echo htmlspecialchars($article['slug']); ?>.php"><?php echo htmlspecialchars($article['title']); ?></a></h3>
             <span class="read-time"><?php echo htmlspecialchars($article['read_time']); ?></span>
         </article>
         <?php endforeach; ?>