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>

single.php

@@ -14,6 +14,8 @@ get_header();
 
 <?php while (have_posts()) : the_post(); ?>
 
+<main id="main-content" class="site-main" role="main">
+
 <!-- Hero Section - Componente dinámico -->
 <?php
 if (function_exists('roi_render_component')) {
@@ -92,6 +94,8 @@ if (function_exists('roi_render_component')) {
 	</div><!-- .row -->
 </div><!-- .container -->
 
+</main><!-- #main-content -->
+
 <?php endwhile; ?>
 
 <!-- Contact Form Section - Componente dinámico -->