fix: cart payment/order currency ignored the selected currency

createPayment() and recordOrder() hardcoded currency: 'RUB' regardless
of LanguageService.currentCurrency() (app supports RUB/USD/EUR/AMD).
Widened CartPaymentRequest.currency from a 'RUB' literal to string and
use the actual selected currency in both calls.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/services/api.service.ts

@@ -42,7 +42,7 @@ export interface QrCreateResponse {
 
 export interface CartPaymentRequest {
   amount: number;
-  currency: 'RUB';
+  currency: string;
   siteuserID: string;
   siteorderID: string;
   redirectUrl: string;