feat: Improve currency conversion logic in BillingManager to handle multiple currencies and enhance accuracy in RUB calculations

frontend/src/BillingManager.jsx

@@ -292,8 +292,18 @@ function BillingManager() {
 
   function convertToRUB(amount, currency) {
     if (!amount) return 0;
-    const rate = exchangeRates[currency] || 1;
-    return amount * rate * exchangeRates.RUB;
+    // Курсы получены с базой USD (1 USD = rates[CCY])
+    const rubPerUsd = Number(exchangeRates.RUB) || 1;
+    if (!currency || currency === 'RUB') return amount; // уже в рублях
+    if (currency === 'USD') return amount * rubPerUsd;
+    const ccyPerUsd = Number(exchangeRates[currency]);
+    if (!ccyPerUsd || ccyPerUsd === 0) {
+      // нет курса — возвращаем как есть
+      return amount;
+    }
+    // amount в валюте CCY -> USD -> RUB
+    const amountUsd = amount / ccyPerUsd;
+    return amountUsd * rubPerUsd;
   }
 
   function formatCurrencyInRUB(amount, currency) {