fix(orders): restore frontend pay REST and harden order-center client

Add AKSRT\Payment\OrderCenterRest so /shop/frontend/orders/{n}/pay|cancel
always register on rest_api_init; widen order_number route charset; make
order-center.js use credentials, encodeURIComponent, and clearer 404 errors.

functions.php

@@ -223,6 +223,9 @@ if (class_exists('\AKSRT\Content\VipService')) {
 if (class_exists('\AKSRT\Payment\ChargeCard')) {
     \AKSRT\Payment\ChargeCard::get_instance()->init();
 }
+if (class_exists('\AKSRT\Payment\OrderCenterRest')) {
+    \AKSRT\Payment\OrderCenterRest::get_instance()->init();
+}
 if (class_exists('\AKSRT\Credits\BalanceService')) {
     \AKSRT\Credits\BalanceService::get_instance()->init();
 }