feat: client-side currency conversion with admin-configurable rates

- CurrencyRatesService: RUB-based rates, persisted via localStorage
- CurrencyConvertPipe: impure pipe converting item price to selected currency
- Admin settings page: editable currency rates form
- Applied conversion to product-card, product-information, quick-view-dialog,
  delivery-information, compare-table, cart totals + payment payload

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

src/app/i18n/en.ts

@@ -1947,6 +1947,10 @@ export const en: Translations = {
     density: 'Table density',
     densityExplain: 'Reduce row padding across backoffice list pages for a more compact view.',
     densityCompact: 'Compact rows',
+    currencyRates: 'Currency rates',
+    currencyRatesExplain: 'Rates relative to 1 RUB, used to convert storefront prices while the backend does not return prices per currency.',
+    currencyRatesSave: 'Save rates',
+    currencyRatesSaved: 'Rates saved',
   },
   adminAnalytics: {
     topProductsEmptyTitle: 'No product sales in this period',