feat: optionally create missing WooCommerce products from BC items

The plugin only ever updated existing WooCommerce products - nothing
created new ones, so a category-filtered sync would just skip every BC
item without a matching product, no matter how the filter was configured.

Adds an opt-in "Create Missing Products" setting (default off, so existing
update-only behavior is unchanged unless explicitly enabled) that creates
a bare WC_Product_Simple (name, SKU from GTIN/number, configurable
draft/publish status) for unmatched BC items in the category-filtered
sync path, then runs it straight through the existing update_product()
call for stock/price/brand - no duplicated logic.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -89,6 +89,12 @@ class WBC_Admin {
         register_setting( 'wbc_sync', 'wbc_category_parent_map', array(
             'sanitize_callback' => 'sanitize_textarea_field',
         ) );
+        register_setting( 'wbc_sync', 'wbc_create_missing_products', array(
+            'sanitize_callback' => array( $this, 'sanitize_checkbox' ),
+        ) );
+        register_setting( 'wbc_sync', 'wbc_new_product_status', array(
+            'sanitize_callback' => array( $this, 'sanitize_new_product_status' ),
+        ) );
 
         // Order settings (own group)
         register_setting( 'wbc_orders', 'wbc_enable_order_sync', array(
@@ -173,6 +179,16 @@ class WBC_Admin {
         return $value === 'yes' ? 'yes' : 'no';
     }
 
+    /**
+     * Sanitize the new-product status setting
+     *
+     * @param string $value Input value.
+     * @return string 'publish' or 'draft'.
+     */
+    public function sanitize_new_product_status( $value ) {
+        return $value === 'publish' ? 'publish' : 'draft';
+    }
+
     /**
      * Enqueue admin scripts
      *