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/types/index.ts

@@ -343,6 +343,8 @@ export interface Tariff {
   custom_days_discount_percent?: number;
   // Traffic reset
   traffic_reset_mode?: string;
+  // Multi-tariff: already purchased by user
+  is_purchased?: boolean;
 }
 
 export interface TariffsPurchaseOptions {
@@ -355,6 +357,8 @@ export interface TariffsPurchaseOptions {
   subscription_status?: string;
   subscription_is_expired?: boolean;
   has_subscription?: boolean;
+  // Multi-tariff: all available tariffs already purchased
+  all_tariffs_purchased?: boolean;
 }
 
 export interface ClassicPurchaseOptions {