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/translations.ts

@@ -1955,6 +1955,10 @@ export interface Translations {
     density: string;
     densityExplain: string;
     densityCompact: string;
+    currencyRates: string;
+    currencyRatesExplain: string;
+    currencyRatesSave: string;
+    currencyRatesSaved: string;
   };
   adminAnalytics: {
     topProductsEmptyTitle: string;