feat: customer lookup by NIF with default customer fallback

- Search BC customers by taxRegistrationNumber (NIF/CIF from _nif meta)
- If NIF found in BC, use that customer; if not, create with NIF
- If no NIF on order, use configurable default customer number
- Add Default Customer Number setting in Order Settings tab

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

woo-business-central/admin/class-wbc-admin.php

@@ -94,6 +94,9 @@ class WBC_Admin {
         register_setting( 'wbc_orders', 'wbc_shipping_item_number', array(
             'sanitize_callback' => 'sanitize_text_field',
         ) );
+        register_setting( 'wbc_orders', 'wbc_default_customer_number', array(
+            'sanitize_callback' => 'sanitize_text_field',
+        ) );
     }
 
     /**