fix(format): convert price by exchange rate on landings and gift page

formatPrice from utils/format.ts only swapped the currency symbol
(220 ₽ -> ¥220) without converting the underlying amount, because it
had no source of exchange rates and the landing/gift pages never called
the useCurrency hook that knew how to fetch them.

- Add a module-level rates cache in utils/format with setExchangeRates
  setter. formatPrice now converts kopeks/100 from RUB to the target
  currency via currencyApi.convertFromRub when rates are cached.
- useCurrency pushes its loaded rates into that cache so any subsequent
  formatPrice call benefits, including subcomponents that cannot easily
  receive a prop.
- Call useCurrency in QuickPurchase, GiftSubscription, and
  AdminLandingEditor — the only entry points whose subcomponents still
  use the synchronous formatPrice (everything else already routes
  through useCurrency.formatAmount).

For RU locale behavior is unchanged. For EN/ZH/FA the amount is now
divided by the per-currency rate and formatted via Intl.NumberFormat
with 2 fraction digits (0 for IRR since amounts are large).

src/pages/AdminLandingEditor.tsx

@@ -13,6 +13,7 @@ import {
 } from '../api/landings';
 import { tariffsApi, TariffListItem, PeriodPrice } from '../api/tariffs';
 import { formatPrice } from '../utils/format';
+import { useCurrency } from '../hooks/useCurrency';
 import { adminPaymentMethodsApi } from '../api/adminPaymentMethods';
 import { Toggle, LocaleTabs, LocalizedInput } from '../components/admin';
 import { BackgroundConfigEditor } from '../components/admin/BackgroundConfigEditor';
@@ -97,6 +98,9 @@ export default function AdminLandingEditor() {
   const { capabilities } = usePlatform();
   const isEdit = !!id;
 
+  // Прогреваем кэш курсов валют для formatPrice (preview лендинга в не-RU локали).
+  useCurrency();
+
   // Section visibility
   const [openSections, setOpenSections] = useState<Record<string, boolean>>({
     general: true,