feat: separate renewal and purchase flows in multi-tariff mode

Renewal flow (per-subscription):
- New /subscriptions/:id/renew page showing only period options for
  the specific tariff with balance display and renew button
- PurchaseCTAButton links to /renew in multi-tariff mode

Purchase flow (new tariffs only):
- Hide already purchased tariffs (is_purchased flag from backend)
- Fallback UI when all tariffs are owned
- Types: added is_purchased, all_tariffs_purchased fields

src/pages/Subscription.tsx

@@ -1257,7 +1257,7 @@ export default function Subscription() {
       )}
 
       {/* Purchase / Renewal CTA */}
-      <PurchaseCTAButton subscription={subscription} />
+      <PurchaseCTAButton subscription={subscription} isMultiTariff={isMultiTariff} />
 
       {/* Additional Options (Buy Devices) */}
       {subscription &&