feat: add form builder AJAX handlers (F2.11)

- Create FormBuilderAjax class for React form builder
- Add formipay_save_form_fields AJAX action
- Add formipay_load_form_fields AJAX action
- Sanitize field data on save
- Update FormBuilder to load fields on mount
- Add save status feedback (saving, saved, error)
- Register FormBuilderAjax singleton in main plugin file

formipay.php

@@ -50,6 +50,7 @@ spl_autoload_register(function ($class) {
 
 \Formipay\Init::get_instance();
 \Formipay\Admin\ReactAdmin::get_instance();
+\Formipay\Admin\FormBuilderAjax::get_instance();
 
 register_activation_hook( __FILE__, 'formipay_activate' );
 function formipay_activate() {