fix(ns): class_exists() must use FQCN after PSR-4

Bare class_exists('Balance') looks up global Balance, always false
under AKSRT\* namespaces → REST 403 on balance exchange and friends.
Rewrite string checks to AKSRT\{Domain}\{Class}.

src/AKSRT/Shop/ProductReview.php

@@ -275,7 +275,7 @@ class ProductReview {
             $order_number = (string) ($order['order_number'] ?? '');
             if ($user_id <= 0 || $order_number === '') continue;
 
-            $items = class_exists('Shop')
+            $items = class_exists('AKSRT\\Shop\\Shop')
                 ? Shop::get_instance()->get_order_items($order_number)
                 : [];