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/en.ts

@@ -124,6 +124,7 @@ export const en: Translations = {
     emailNeedsDomain: 'Email must contain a domain (.com, .ru, etc.)',
     emailInvalid: 'Invalid email format',
     telegramIdMissing: 'We could not identify your Telegram account, so we could not save your contact details. Your payment was still successful.',
+    paymentDescriptionFallback: 'Purchase on Marketplace',
     emailPlaceholder: 'you@example.com',
     phonePlaceholder: '+7 (___) ___-__-__',
     loginRequired: 'Log in to checkout',