fix(accessibility): Add main landmark and fix cta-box-title heading

Phase 4.4 Accessibility fixes:
- single.php: Add <main id="main-content" role="main"> landmark
- CtaBoxSidebarRenderer: h5 cta-box-title changed to span

Fixes: "No main landmark" and "Headings skip levels"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Public/CtaBoxSidebar/Infrastructure/Ui/CtaBoxSidebarRenderer.php

@@ -182,9 +182,9 @@ final class CtaBoxSidebarRenderer implements RendererInterface
 
         $html = '<div class="cta-box-sidebar">';
 
-        // Title
+        // Title (span instead of h5 for semantic heading hierarchy - Phase 4.4 Accessibility)
         $html .= sprintf(
-            '<h5 class="cta-box-title">%s</h5>',
+            '<span class="cta-box-title d-block h5">%s</span>',
             esc_html($title)
         );