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/Footer/Infrastructure/FieldMapping/FooterFieldMapper.php

@@ -0,0 +1,75 @@
+<?php
+declare(strict_types=1);
+
+namespace ROITheme\Admin\Footer\Infrastructure\FieldMapping;
+
+use ROITheme\Admin\Shared\Domain\Contracts\FieldMapperInterface;
+
+/**
+ * Field Mapper para Footer
+ *
+ * RESPONSABILIDAD:
+ * - Mapear field IDs del formulario a atributos de BD
+ * - Solo conoce sus propios campos (modularidad)
+ */
+final class FooterFieldMapper implements FieldMapperInterface
+{
+    public function getComponentName(): string
+    {
+        return 'footer';
+    }
+
+    public function getFieldMapping(): array
+    {
+        return [
+            // Visibility
+            'footerEnabled' => ['group' => 'visibility', 'attribute' => 'is_enabled'],
+            'footerShowOnDesktop' => ['group' => 'visibility', 'attribute' => 'show_on_desktop'],
+            'footerShowOnMobile' => ['group' => 'visibility', 'attribute' => 'show_on_mobile'],
+
+            // Widget 1
+            'footerWidget1Visible' => ['group' => 'widget_1', 'attribute' => 'widget_1_visible'],
+            'footerWidget1Title' => ['group' => 'widget_1', 'attribute' => 'widget_1_title'],
+
+            // Widget 2
+            'footerWidget2Visible' => ['group' => 'widget_2', 'attribute' => 'widget_2_visible'],
+            'footerWidget2Title' => ['group' => 'widget_2', 'attribute' => 'widget_2_title'],
+
+            // Widget 3
+            'footerWidget3Visible' => ['group' => 'widget_3', 'attribute' => 'widget_3_visible'],
+            'footerWidget3Title' => ['group' => 'widget_3', 'attribute' => 'widget_3_title'],
+
+            // Newsletter
+            'footerNewsletterVisible' => ['group' => 'newsletter', 'attribute' => 'newsletter_visible'],
+            'footerNewsletterTitle' => ['group' => 'newsletter', 'attribute' => 'newsletter_title'],
+            'footerNewsletterDescription' => ['group' => 'newsletter', 'attribute' => 'newsletter_description'],
+            'footerNewsletterPlaceholder' => ['group' => 'newsletter', 'attribute' => 'newsletter_email_placeholder'],
+            'footerNewsletterButtonText' => ['group' => 'newsletter', 'attribute' => 'newsletter_button_text'],
+            'footerNewsletterWebhookUrl' => ['group' => 'newsletter', 'attribute' => 'newsletter_webhook_url'],
+            'footerNewsletterSuccessMessage' => ['group' => 'newsletter', 'attribute' => 'newsletter_success_message'],
+            'footerNewsletterErrorMessage' => ['group' => 'newsletter', 'attribute' => 'newsletter_error_message'],
+
+            // Footer Bottom
+            'footerCopyrightText' => ['group' => 'footer_bottom', 'attribute' => 'copyright_text'],
+
+            // Colors
+            'footerBgColor' => ['group' => 'colors', 'attribute' => 'bg_color'],
+            'footerTextColor' => ['group' => 'colors', 'attribute' => 'text_color'],
+            'footerTitleColor' => ['group' => 'colors', 'attribute' => 'title_color'],
+            'footerLinkColor' => ['group' => 'colors', 'attribute' => 'link_color'],
+            'footerLinkHoverColor' => ['group' => 'colors', 'attribute' => 'link_hover_color'],
+            'footerButtonBgColor' => ['group' => 'colors', 'attribute' => 'button_bg_color'],
+            'footerButtonTextColor' => ['group' => 'colors', 'attribute' => 'button_text_color'],
+            'footerButtonHoverBg' => ['group' => 'colors', 'attribute' => 'button_hover_bg'],
+
+            // Spacing
+            'footerPaddingY' => ['group' => 'spacing', 'attribute' => 'padding_y'],
+            'footerMarginTop' => ['group' => 'spacing', 'attribute' => 'margin_top'],
+
+            // Visual Effects
+            'footerInputBorderRadius' => ['group' => 'visual_effects', 'attribute' => 'input_border_radius'],
+            'footerButtonBorderRadius' => ['group' => 'visual_effects', 'attribute' => 'button_border_radius'],
+            'footerTransitionDuration' => ['group' => 'visual_effects', 'attribute' => 'transition_duration'],
+        ];
+    }
+}