fix(navigation): break BackButton loop on single-tariff /subscriptions/:id deep-link

User flow that hung:
  1. Bot deep-link opens MiniApp directly on /subscriptions/:id (idx=0).
  2. Telegram shows BackButton.
  3. Tap → TelegramBackButton handler sees no in-app history and falls
     back to getFallbackParentPath('/subscriptions/:id') = '/subscriptions'.
  4. Subscriptions page renders, sees single-tariff + 1 subscription,
     <Navigate replace> redirects right back to /subscriptions/:id.
  5. BackButton looks dead — taps do nothing visible.

Fix: only redirect when there is in-app history. When idx=0 (deep-link or
just-arrived-via-back), render the list page instead. The user then sees
their single subscription as a card and can tap into it, and the next
BackButton tap exits the MiniApp as expected.

Reported by Серёжа in bugs topic #599679.

src/pages/Subscriptions.tsx

@@ -9,6 +9,7 @@ import { getGlassColors } from '../utils/glassTheme';
 import { useAuthStore } from '../store/auth';
 import SubscriptionListCard from '../components/subscription/SubscriptionListCard';
 import TrialOfferCard from '../components/dashboard/TrialOfferCard';
+import { hasInAppHistory } from '../utils/navigation';
 
 function EmptyState({ onBuy }: { onBuy: () => void }) {
   const { t } = useTranslation();
@@ -106,8 +107,11 @@ export default function Subscriptions() {
     },
   });
 
-  // Single-tariff mode with one subscription: skip list, go directly to detail
+  // Single-tariff mode with one subscription: skip list, go directly to detail.
-  if (data && !isMultiTariff && subscriptions.length === 1) {
+  // Skip the redirect when the user just hit the Telegram BackButton from
+  // /subscriptions/:id — that lands us here with idx=0, and redirecting back
+  // to the detail page creates a Back-button loop the user can't escape (#599679).
+  if (data && !isMultiTariff && subscriptions.length === 1 && hasInAppHistory()) {
     return <Navigate to={`/subscriptions/${subscriptions[0].id}`} replace />;
   }