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/Shortcodes.php

@@ -260,7 +260,7 @@ class Shortcodes {
      * [aksrt_archive] 文章归档
      */
     public function render_archive(array $atts = [], string $content = ''): string {
-        if (!class_exists('Templates')) {
+        if (!class_exists('AKSRT\\Core\\Templates')) {
             return '';
         }
         return Templates::get_instance()->render_archive_shortcode($atts, $content);
@@ -270,7 +270,7 @@ class Shortcodes {
      * [aksrt_profile] 个人资料卡
      */
     public function render_profile(array $atts = [], string $content = ''): string {
-        if (!class_exists('Templates')) {
+        if (!class_exists('AKSRT\\Core\\Templates')) {
             return '';
         }
         return Templates::get_instance()->render_profile_shortcode($atts, $content);