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/nav.php

@@ -13,7 +13,7 @@ function nav_active($href, $current) {
 }
 ?>
 <!-- Navigation -->
-<nav class="navbar" id="navbar">
+<nav class="navbar" id="navbar" aria-label="Main navigation">
     <div class="nav-container">
         <div class="nav-logo">
             <a href="/">