feat(pay): VIP package purchase service and user-center open flow

Add AKSRT_Vip_Service (GET/POST /pay/vip), activate aksrt_vip_level/expires on
order paid, package UI in credits panel, and deep-link VIP CTA to credits tab.

functions.php

@@ -177,6 +177,9 @@ if (class_exists('AKSRT_Download')) {
 if (class_exists('AKSRT_Content_Pay')) {
     AKSRT_Content_Pay::get_instance()->init();
 }
+if (class_exists('AKSRT_Vip_Service')) {
+    AKSRT_Vip_Service::get_instance()->init();
+}
 // Download State & Docs Presenter are pure service singletons (no hooks)
 if (class_exists('AKSRT_Download_State')) {
     AKSRT_Download_State::get_instance();