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/Core/ResourceMetaBox.php

@@ -486,7 +486,7 @@ class ResourceMetaBox {
         $gallery = $this->parse_json_array($_POST['aksrt_pay_gallery'] ?? '');
 
         // 清洗下载源
-        if ($downloads !== null && class_exists('Setup') && method_exists('Setup', 'sanitize_downloads_meta')) {
+        if ($downloads !== null && class_exists('AKSRT\\Core\\Setup') && method_exists('AKSRT\\Core\\Setup', 'sanitize_downloads_meta')) {
             $downloads = Setup::sanitize_downloads_meta($downloads);
         }