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/pages/cart/cart.component.ts

@@ -256,7 +256,7 @@ export class CartComponent implements OnDestroy {
     const orderId = this.generateOrderId();
     const paymentPayload = {
       amount: Number(this.totalWithDelivery()),
-      currency: 'RUB' as const,
+      currency: this.langService.currentCurrency(),
       siteuserID: this.getPaymentUserId(),
       siteorderID: orderId,
       redirectUrl: '',
@@ -434,7 +434,7 @@ export class CartComponent implements OnDestroy {
       },
       payment: {
         method: this.selectedPaymentMethod(),
-        currency: 'RUB',
+        currency: this.langService.currentCurrency(),
       },
     }).subscribe({
       error: (err) => console.error('Error recording order:', err),