fix: show missing balance amount on renew page, no tariff switch in multi-tariff

- RenewSubscription: display actual missing amount instead of {{missing}} placeholder
- SubscriptionPurchase: suppress "switch tariff" title in multi-tariff mode
  (only renewal exists, no tariff switching)

src/pages/SubscriptionPurchase.tsx

@@ -411,7 +411,7 @@ export default function SubscriptionPurchase() {
       <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">
         {isMultiTariff && !subscriptionId
           ? t('subscription.newTariff', 'Новый тариф')
-          : subscription?.is_daily && !subscription?.is_trial
+          : !isMultiTariff && subscription?.is_daily && !subscription?.is_trial
             ? t('subscription.switchTariff.title')
             : subscription && !subscription.is_trial
               ? t('subscription.extend')