refactor(admin): Migrate AdminAjaxHandler to Clean Architecture

- Move AdminAjaxHandler to Admin/Shared/Infrastructure/Api/Wordpress/
- Create FieldMapperInterface for decentralized field mapping
- Create FieldMapperRegistry for module discovery
- Create FieldMapperProvider for auto-registration of 12 mappers
- Add FieldMappers for all components:
  - ContactFormFieldMapper (46 fields)
  - CtaBoxSidebarFieldMapper (32 fields)
  - CtaLetsTalkFieldMapper
  - CtaPostFieldMapper
  - FeaturedImageFieldMapper (15 fields)
  - FooterFieldMapper (31 fields)
  - HeroFieldMapper
  - NavbarFieldMapper
  - RelatedPostFieldMapper (34 fields)
  - SocialShareFieldMapper
  - TableOfContentsFieldMapper
  - TopNotificationBarFieldMapper (17 fields)
- Update functions.php bootstrap with FieldMapperProvider
- AdminAjaxHandler reduced from ~700 to 145 lines
- Follows SRP, OCP, DIP principles

BACKUP BEFORE: Removing CTA A/B Testing legacy system

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

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

Admin/RelatedPost/Infrastructure/FieldMapping/RelatedPostFieldMapper.php

@@ -0,0 +1,77 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Admin\RelatedPost\Infrastructure\FieldMapping;
+
+use ROITheme\Admin\Shared\Domain\Contracts\FieldMapperInterface;
+
+/**
+ * Field Mapper para Related Post
+ *
+ * RESPONSABILIDAD:
+ * - Mapear field IDs del formulario a atributos de BD
+ * - Solo conoce sus propios campos (modularidad)
+ *
+ * NOTA: Este componente NO tenia mapeos en AdminAjaxHandler
+ * (era el unico componente roto - 35 campos no se guardaban)
+ */
+final class RelatedPostFieldMapper implements FieldMapperInterface
+{
+    public function getComponentName(): string
+    {
+        return 'related-post';
+    }
+
+    public function getFieldMapping(): array
+    {
+        return [
+            // Visibility
+            'relatedPostEnabled' => ['group' => 'visibility', 'attribute' => 'is_enabled'],
+            'relatedPostShowOnDesktop' => ['group' => 'visibility', 'attribute' => 'show_on_desktop'],
+            'relatedPostShowOnMobile' => ['group' => 'visibility', 'attribute' => 'show_on_mobile'],
+            'relatedPostShowOnPages' => ['group' => 'visibility', 'attribute' => 'show_on_pages'],
+
+            // Content
+            'relatedPostSectionTitle' => ['group' => 'content', 'attribute' => 'section_title'],
+            'relatedPostPerPage' => ['group' => 'content', 'attribute' => 'posts_per_page'],
+            'relatedPostOrderby' => ['group' => 'content', 'attribute' => 'orderby'],
+            'relatedPostOrder' => ['group' => 'content', 'attribute' => 'order'],
+            'relatedPostShowPagination' => ['group' => 'content', 'attribute' => 'show_pagination'],
+
+            // Layout
+            'relatedPostColsDesktop' => ['group' => 'layout', 'attribute' => 'columns_desktop'],
+            'relatedPostColsTablet' => ['group' => 'layout', 'attribute' => 'columns_tablet'],
+            'relatedPostColsMobile' => ['group' => 'layout', 'attribute' => 'columns_mobile'],
+
+            // Typography
+            'relatedPostSectionTitleSize' => ['group' => 'typography', 'attribute' => 'section_title_size'],
+            'relatedPostSectionTitleWeight' => ['group' => 'typography', 'attribute' => 'section_title_weight'],
+            'relatedPostCardTitleSize' => ['group' => 'typography', 'attribute' => 'card_title_size'],
+            'relatedPostCardTitleWeight' => ['group' => 'typography', 'attribute' => 'card_title_weight'],
+
+            // Colors
+            'relatedPostSectionTitleColor' => ['group' => 'colors', 'attribute' => 'section_title_color'],
+            'relatedPostCardBgColor' => ['group' => 'colors', 'attribute' => 'card_bg_color'],
+            'relatedPostCardTitleColor' => ['group' => 'colors', 'attribute' => 'card_title_color'],
+            'relatedPostCardHoverBgColor' => ['group' => 'colors', 'attribute' => 'card_hover_bg_color'],
+            'relatedPostPaginationBgColor' => ['group' => 'colors', 'attribute' => 'pagination_bg_color'],
+            'relatedPostPaginationTextColor' => ['group' => 'colors', 'attribute' => 'pagination_text_color'],
+            'relatedPostPaginationActiveBg' => ['group' => 'colors', 'attribute' => 'pagination_active_bg'],
+            'relatedPostPaginationActiveText' => ['group' => 'colors', 'attribute' => 'pagination_active_text'],
+
+            // Spacing
+            'relatedPostSectionMarginTop' => ['group' => 'spacing', 'attribute' => 'section_margin_top'],
+            'relatedPostSectionMarginBottom' => ['group' => 'spacing', 'attribute' => 'section_margin_bottom'],
+            'relatedPostTitleMarginBottom' => ['group' => 'spacing', 'attribute' => 'title_margin_bottom'],
+            'relatedPostGridGap' => ['group' => 'spacing', 'attribute' => 'grid_gap'],
+            'relatedPostCardPadding' => ['group' => 'spacing', 'attribute' => 'card_padding'],
+            'relatedPostPaginationMarginTop' => ['group' => 'spacing', 'attribute' => 'pagination_margin_top'],
+
+            // Visual Effects
+            'relatedPostCardBorderRadius' => ['group' => 'visual_effects', 'attribute' => 'card_border_radius'],
+            'relatedPostCardShadow' => ['group' => 'visual_effects', 'attribute' => 'card_shadow'],
+            'relatedPostCardHoverShadow' => ['group' => 'visual_effects', 'attribute' => 'card_hover_shadow'],
+            'relatedPostCardTransition' => ['group' => 'visual_effects', 'attribute' => 'card_transition'],
+        ];
+    }
+}