refactor(purchase): extract usePromoDiscount hook

Prerequisite for the SubscriptionPurchase god-page decomposition.
The applyPromoDiscount helper + its active-discount useQuery were
inlined in SubscriptionPurchase.tsx and called from 9 sites across
3 purchase flows (classic wizard, tariff picker, tariff form,
switch-tariff sheet). Lifting both into src/hooks/usePromoDiscount
gives every future extracted sub-component a single seam to call,
without threading the function through props or re-fetching the
discount in each child.

SubscriptionPurchase.tsx: 2054 → 2009 lines (-45).

src/hooks/usePromoDiscount.ts

@@ -0,0 +1,73 @@
+import { useCallback } from 'react';
+import { useQuery } from '@tanstack/react-query';
+import { promoApi } from '../api/promo';
+
+// ──────────────────────────────────────────────────────────────────
+// usePromoDiscount
+//
+// Single source of truth for the active-discount query + the
+// applyPromoDiscount helper. Extracted from SubscriptionPurchase.tsx
+// so that every flow / sub-component (tariff picker, tariff purchase
+// form, classic wizard, switch-tariff sheet) can call the same hook
+// without re-fetching or threading a function through props.
+//
+// Returns:
+//   activeDiscount: the raw API value (or undefined while loading)
+//   applyPromoDiscount: combines the active discount with any
+//     pre-existing price reduction (promo-group pricing) and reports
+//     final price, original price, total percent off, and whether
+//     the existing reduction is a promo-group price.
+// ──────────────────────────────────────────────────────────────────
+
+export interface PromoDiscountResult {
+  price: number;
+  original: number | null;
+  percent: number | null;
+  isPromoGroup: boolean;
+}
+
+export function usePromoDiscount() {
+  const { data: activeDiscount } = useQuery({
+    queryKey: ['active-discount'],
+    queryFn: promoApi.getActiveDiscount,
+    staleTime: 30000,
+  });
+
+  const applyPromoDiscount = useCallback(
+    (priceKopeks: number, existingOriginalPrice?: number | null): PromoDiscountResult => {
+      const hasExisting = (existingOriginalPrice ?? 0) > priceKopeks;
+      const hasPromo = !!activeDiscount?.is_active && !!activeDiscount.discount_percent;
+
+      if (!hasExisting && !hasPromo) {
+        return { price: priceKopeks, original: null, percent: null, isPromoGroup: false };
+      }
+
+      let finalPrice = priceKopeks;
+      if (hasPromo) {
+        finalPrice = Math.round(priceKopeks * (1 - activeDiscount!.discount_percent! / 100));
+      }
+
+      if (hasExisting) {
+        const combinedPercent = hasPromo
+          ? Math.round((1 - finalPrice / existingOriginalPrice!) * 100)
+          : Math.round((1 - priceKopeks / existingOriginalPrice!) * 100);
+        return {
+          price: finalPrice,
+          original: existingOriginalPrice!,
+          percent: combinedPercent,
+          isPromoGroup: true,
+        };
+      }
+
+      return {
+        price: finalPrice,
+        original: priceKopeks,
+        percent: activeDiscount!.discount_percent!,
+        isPromoGroup: false,
+      };
+    },
+    [activeDiscount],
+  );
+
+  return { activeDiscount, applyPromoDiscount };
+}

src/pages/SubscriptionPurchase.tsx

@@ -4,7 +4,7 @@ import { useTranslation } from 'react-i18next';
 import { useNavigate, useSearchParams } from 'react-router';
 import { AxiosError } from 'axios';
 import { subscriptionApi } from '../api/subscription';
-import { promoApi } from '../api/promo';
+import { usePromoDiscount } from '../hooks/usePromoDiscount';
 import { WebBackButton } from '../components/WebBackButton';
 import { getGlassColors } from '../utils/glassTheme';
 import { useTheme } from '../hooks/useTheme';
@@ -66,12 +66,8 @@ export default function SubscriptionPurchase() {
     refetchOnMount: 'always',
   });
 
-  // Active promo discount
-  const { data: activeDiscount } = useQuery({
-    queryKey: ['active-discount'],
-    queryFn: promoApi.getActiveDiscount,
-    staleTime: 30000,
-  });
+  // Active promo discount + applyPromoDiscount helper (hook owns both)
+  const { activeDiscount, applyPromoDiscount } = usePromoDiscount();
 
   // Sales mode detection
   const isTariffsMode = purchaseOptions?.sales_mode === 'tariffs';
@@ -87,47 +83,7 @@ export default function SubscriptionPurchase() {
   });
   const isMultiTariff = multiSubData?.multi_tariff_enabled ?? false;
 
-  // Helper to apply promo discount
-  const applyPromoDiscount = (
-    priceKopeks: number,
-    existingOriginalPrice?: number | null,
-  ): {
-    price: number;
-    original: number | null;
-    percent: number | null;
-    isPromoGroup: boolean;
-  } => {
-    const hasExisting = (existingOriginalPrice ?? 0) > priceKopeks;
-    const hasPromo = !!activeDiscount?.is_active && !!activeDiscount.discount_percent;
-
-    if (!hasExisting && !hasPromo) {
-      return { price: priceKopeks, original: null, percent: null, isPromoGroup: false };
-    }
-
-    let finalPrice = priceKopeks;
-    if (hasPromo) {
-      finalPrice = Math.round(priceKopeks * (1 - activeDiscount!.discount_percent! / 100));
-    }
-
-    if (hasExisting) {
-      const combinedPercent = hasPromo
-        ? Math.round((1 - finalPrice / existingOriginalPrice!) * 100)
-        : Math.round((1 - priceKopeks / existingOriginalPrice!) * 100);
-      return {
-        price: finalPrice,
-        original: existingOriginalPrice!,
-        percent: combinedPercent,
-        isPromoGroup: true,
-      };
-    }
-
-    return {
-      price: finalPrice,
-      original: priceKopeks,
-      percent: activeDiscount!.discount_percent!,
-      isPromoGroup: false,
-    };
-  };
+  // (applyPromoDiscount moved into usePromoDiscount above)
 
   // Classic mode state
   const [currentStep, setCurrentStep] = useState<PurchaseStep>('period');