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/Messages/PrivateMessage.php

@@ -184,7 +184,7 @@ class PrivateMessage {
 
         // Create notification for the recipient
         $recipient_id = $is_a ? (int) $conv['user_b'] : (int) $conv['user_a'];
-        if (class_exists('Message')) {
+        if (class_exists('AKSRT\\Messages\\Message')) {
             $sender_name = '';
             $sender_user = get_userdata($sender_id);
             if ($sender_user) {