fix: show error state instead of blank page on purchase-options failure

When the purchase-options API call failed (e.g. for email-registered
users), the page rendered only the header with no content. Now shows
an error message with retry button. Also adds a fallback for when
the API returns no available tariffs or periods.

src/pages/SubscriptionPurchase.tsx

@@ -45,7 +45,12 @@ export default function SubscriptionPurchase() {
   const subscription = subscriptionResponse?.subscription ?? null;
 
   // Purchase options
-  const { data: purchaseOptions, isLoading: optionsLoading } = useQuery({
+  const {
+    data: purchaseOptions,
+    isLoading: optionsLoading,
+    isError: optionsError,
+    refetch: refetchOptions,
+  } = useQuery({
     queryKey: ['purchase-options'],
     queryFn: subscriptionApi.getPurchaseOptions,
     staleTime: 0,
@@ -362,6 +367,58 @@ export default function SubscriptionPurchase() {
     );
   }
 
+  if (optionsError || (!purchaseOptions && !optionsLoading)) {
+    return (
+      <div className="space-y-6">
+        <div className="flex items-center gap-3">
+          <button
+            onClick={() => navigate('/subscription')}
+            aria-label="Back"
+            className="flex h-9 w-9 items-center justify-center rounded-xl transition-colors"
+            style={{
+              background: g.innerBg,
+              border: `1px solid ${g.innerBorder}`,
+            }}
+          >
+            <svg
+              width="18"
+              height="18"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              strokeWidth="2"
+              strokeLinecap="round"
+              strokeLinejoin="round"
+              className="text-dark-50/60"
+            >
+              <path d="M19 12H5M12 19l-7-7 7-7" />
+            </svg>
+          </button>
+          <h1 className="text-2xl font-bold text-dark-50 sm:text-3xl">
+            {t('subscription.extend')}
+          </h1>
+        </div>
+        <div
+          className="rounded-3xl p-6 text-center"
+          style={{
+            background: g.cardBg,
+            border: `1px solid ${g.cardBorder}`,
+          }}
+        >
+          <p className="mb-4 text-dark-300">
+            {t('subscription.loadError', 'Не удалось загрузить варианты подписки')}
+          </p>
+          <button
+            onClick={() => refetchOptions()}
+            className="rounded-xl bg-accent-500 px-6 py-2 text-sm font-medium text-white transition-colors hover:bg-accent-600"
+          >
+            {t('common.retry')}
+          </button>
+        </div>
+      </div>
+    );
+  }
+
   return (
     <div className="space-y-6">
       {/* Header with back link */}
@@ -1936,6 +1993,30 @@ export default function SubscriptionPurchase() {
           )}
         </div>
       )}
+
+      {/* No options available fallback */}
+      {purchaseOptions &&
+        !optionsLoading &&
+        !(isTariffsMode && tariffs.length > 0) &&
+        !(classicOptions && classicOptions.periods.length > 0) && (
+          <div
+            className="rounded-3xl p-6 text-center"
+            style={{
+              background: g.cardBg,
+              border: `1px solid ${g.cardBorder}`,
+            }}
+          >
+            <p className="mb-4 text-dark-300">
+              {t('subscription.noOptionsAvailable', 'Нет доступных вариантов подписки')}
+            </p>
+            <button
+              onClick={() => refetchOptions()}
+              className="rounded-xl bg-accent-500 px-6 py-2 text-sm font-medium text-white transition-colors hover:bg-accent-600"
+            >
+              {t('common.retry')}
+            </button>
+          </div>
+        )}
     </div>
   );
 }