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)

blog/articles/predictive-analytics-customer-churn.php

@@ -33,7 +33,7 @@ $read_time = 14;
     <meta name="article:tag" content="Predictive Analytics, Customer Churn, Machine Learning, Retention">
     
     <!-- Preload critical resources -->
-    <link rel="preload" href="../../assets/css/main.css" as="style">
+    <link rel="preload" href="../../assets/css/main.css?v=20260222" as="style">
     <link rel="preload" href="../../assets/images/ukds-main-logo.png" as="image">
     
     <!-- Open Graph / Social Media -->
@@ -64,8 +64,8 @@ $read_time = 14;
     <link href="https://fonts.googleapis.com/css2?family=Roboto+Slab:wght@300;400;500;600;700&family=Lato:wght@300;400;500;600;700&display=swap" rel="stylesheet">
     
     <!-- Styles -->
-    <link rel="stylesheet" href="../../assets/css/main.css">
-    <link rel="stylesheet" href="../../assets/css/cro-enhancements.css">
+    <link rel="stylesheet" href="../../assets/css/main.css?v=20260222">
+    <link rel="stylesheet" href="../../assets/css/cro-enhancements.css?v=20260222">
     
     <!-- Article Schema Markup -->
     <script type="application/ld+json">
@@ -182,7 +182,7 @@ $read_time = 14;
                 </header>
                 
                 <!-- Table of Contents -->
-                <nav class="article-toc">
+                <nav class="article-toc" aria-label="Table of contents">
                     <h2>Table of Contents</h2>
                     <ol>
                         <li><a href="#churn-fundamentals">Understanding Customer Churn</a></li>