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>

schemas/contact-form.json

@@ -82,7 +82,7 @@
         "phone_label": {
           "type": "text",
           "label": "Label telefono",
-          "default": "Teléfono",
+          "default": "WhatsApp",
           "editable": true
         },
         "phone_value": {
@@ -262,7 +262,7 @@
         "info_value_color": {
           "type": "color",
           "label": "Color valores info",
-          "default": "#6c757d",
+          "default": "#AAAAAA",
           "editable": true,
           "description": "text-muted"
         },