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 };
+}