fix: remove duplicate back buttons, improve multi-subscription UX

- Remove custom back buttons from Subscription detail and SubscriptionPurchase
  pages — native Telegram mini-app back button already handles navigation
- SubscriptionListCard: distinct trial badge with star icon and amber styling,
  trial cards get amber border/background for visual differentiation
- SubscriptionPurchase: change "Купить тариф" to "Новый тариф" when adding
  a new tariff in multi-subscription mode
- Subscription detail: show tariff name as title in multi-tariff mode

src/pages/Subscription.tsx

@@ -479,34 +479,12 @@ export default function Subscription() {
 
   return (
     <div className="space-y-6">
-      {/* Multi-tariff: back navigation + tariff name */}
-      {isMultiTariff && subscriptionId ? (
-        <div>
-          <button
-            onClick={() => navigate('/subscriptions')}
-            className="mb-2 flex items-center gap-1.5 text-sm text-dark-50/50 transition-colors hover:text-dark-50/80"
-          >
-            <svg
-              width="16"
-              height="16"
-              viewBox="0 0 24 24"
-              fill="none"
-              stroke="currentColor"
-              strokeWidth="2"
-              strokeLinecap="round"
-              strokeLinejoin="round"
-            >
-              <path d="M15 18l-6-6 6-6" />
-            </svg>
-            {t('subscription.backToList', 'Мои подписки')}
-          </button>
-          <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">
-            {subscription?.tariff_name || t('subscription.title')}
-          </h1>
-        </div>
-      ) : (
-        <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">{t('subscription.title')}</h1>
-      )}
+      {/* Page title — native Telegram back button handles navigation */}
+      <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">
+        {isMultiTariff && subscription?.tariff_name
+          ? subscription.tariff_name
+          : t('subscription.title')}
+      </h1>
 
       {/* Current Subscription */}
       {subscription ? (