MYOB Integration for WooCommerce

The plugin currently hardcodes ‘Category’ => null in both class-opmc-myob-item-order.php and class-opmc-myob-item-invoice.php. For MYOB company files that have “Category required for all sales” enabled in Setup > Preferences, this causes all order syncs to fail with error code 100: “Category is required”.
There is no setting in the plugin to configure a default transaction category. The only workaround is to directly edit the plugin files, which is overwritten on every update.
Requested fix: add a “Default Transaction Category” dropdown to the plugin settings page, populated by calling the MYOB GeneralLedger/Category endpoint. When set, pass the selected category UID in the Category field on order and invoice payloads. When left blank, send null as current behaviour. This follows the same pattern already used for the Freight Tax Code setting.