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',
+        ) );
     }
 
     /**

woo-business-central/admin/partials/wbc-admin-display.php

@@ -328,6 +328,19 @@ $tabs = array(
                                 </p>
                             </td>
                         </tr>
+                        <tr>
+                            <th scope="row">
+                                <label for="wbc_default_customer_number"><?php esc_html_e( 'Default Customer Number', 'woo-business-central' ); ?></label>
+                            </th>
+                            <td>
+                                <input type="text" id="wbc_default_customer_number" name="wbc_default_customer_number"
+                                       value="<?php echo esc_attr( get_option( 'wbc_default_customer_number', '' ) ); ?>"
+                                       class="regular-text" />
+                                <p class="description">
+                                    <?php esc_html_e( 'BC customer number to use for orders without NIF/CIF. Orders with NIF will look up or create a specific customer.', 'woo-business-central' ); ?>
+                                </p>
+                            </td>
+                        </tr>
                     </table>
 
                     <p class="submit">