feat: category filter, item reference SKU lookup, brand sync, and on-demand BC write-back

- Filter product sync to an allow-list of BC item category codes
- Look up items by ReferenciasArticulo item reference before falling back to GTIN/item number
- Sync BC item category to product_brand and parent category to product_cat via a new ItemCategoryHierarchy endpoint
- Add on-demand push of product name/photo back to BC (product edit meta box + bulk action), including binary PATCH support for item pictures

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

woo-business-central/includes/class-wbc-loader.php

@@ -113,6 +113,13 @@ class WBC_Loader {
             $this->add_action( 'wp_ajax_wbc_get_companies', $admin, 'ajax_get_companies' );
             $this->add_action( 'admin_init', $admin, 'handle_csv_export' );
 
+            // On-demand push-to-BC (individual product + bulk action)
+            $this->add_action( 'add_meta_boxes', $admin, 'add_product_meta_box' );
+            $this->add_action( 'wp_ajax_wbc_push_single_to_bc', $admin, 'ajax_push_single_to_bc' );
+            $this->add_filter( 'bulk_actions-edit-product', $admin, 'add_bulk_action' );
+            $this->add_filter( 'handle_bulk_actions-edit-product', $admin, 'handle_bulk_action', 10, 3 );
+            $this->add_action( 'admin_notices', $admin, 'bulk_action_admin_notice' );
+
             // Add settings link to plugins page
             $this->add_filter( 'plugin_action_links_' . WBC_PLUGIN_BASENAME, $admin, 'add_settings_link' );
         }