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/i18n/translations.ts

@@ -122,6 +122,7 @@ export interface Translations {
     emailNeedsDomain: string;
     emailInvalid: string;
     telegramIdMissing: string;
+    paymentDescriptionFallback: string;
     emailPlaceholder: string;
     phonePlaceholder: string;
     loginRequired: string;