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/Auth/OAuth.php

@@ -41,7 +41,7 @@ class OAuth {
      */
     public function get_auth_url(string $provider, string $action = 'login'): string {
         if (class_exists('\AKSRT\Settings\Settings') && \AKSRT\Settings\Settings::oauth_proxy_mode() === 'client') {
-            if (class_exists('OAuthProxyClient')) {
+            if (class_exists('AKSRT\\Auth\\OAuthProxyClient')) {
                 return OAuthProxyClient::get_instance()->get_proxy_auth_url($provider, $action);
             }
             return '';