fix: checkout payment-description fallback hardcoded Russian regardless of locale

getPaymentDescription()'s final fallback ('Покупка на Маркетплейсе')
ignored the active language. Moved to a translated key.

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

src/app/pages/cart/cart.component.ts

@@ -615,7 +615,7 @@ export class CartComponent implements OnDestroy {
       return hostname;
     }
 
-    return 'Покупка на Маркетплейсе';
+    return this.i18n.t('cart.paymentDescriptionFallback');
   }
 
   private generateOrderId(): string {