refactor: extract purchase/renewal flow to separate page

Move ~1900 lines of purchase/renewal logic from Subscription.tsx into
a new SubscriptionPurchase.tsx page at /subscription/purchase.

- Create SubscriptionPurchase.tsx with tariffs grid, classic mode wizard,
  switch preview modal, and promo handling
- Create PurchaseCTAButton.tsx with animated gradient border, state-aware
  colors and context-aware text
- Create subscriptionHelpers.ts with shared utilities
- Add CopyIcon to shared icons module
- Register /subscription/purchase route with lazy loading
- Update SubscriptionCardExpired and PromoOffersSection navigators
- Add CTA translation keys to ru.json and en.json
- Remove all scrollToExtend references

src/utils/subscriptionHelpers.ts

@@ -0,0 +1,42 @@
+import { AxiosError } from 'axios';
+import i18n from '../i18n';
+
+export type PurchaseStep = 'period' | 'traffic' | 'servers' | 'devices' | 'confirm';
+
+export const getErrorMessage = (error: unknown): string => {
+  if (error instanceof AxiosError) {
+    const detail = error.response?.data?.detail;
+    if (typeof detail === 'string') return detail;
+    if (typeof detail === 'object' && detail?.message) return detail.message;
+  }
+  if (error instanceof Error) return error.message;
+  return i18n.t('common.error');
+};
+
+export const getInsufficientBalanceError = (
+  error: unknown,
+): { required: number; balance: number; missingAmount?: number } | null => {
+  if (error instanceof AxiosError) {
+    const detail = error.response?.data?.detail;
+    if (
+      typeof detail === 'object' &&
+      (detail?.code === 'insufficient_balance' || detail?.code === 'insufficient_funds')
+    ) {
+      return {
+        required: detail.required || detail.total_price || 0,
+        balance: detail.balance || 0,
+        missingAmount: detail.missing_amount || detail.missingAmount || 0,
+      };
+    }
+  }
+  return null;
+};
+
+export const getFlagEmoji = (countryCode: string): string => {
+  if (!countryCode || countryCode.length !== 2) return '';
+  const codePoints = countryCode
+    .toUpperCase()
+    .split('')
+    .map((char) => 127397 + char.charCodeAt(0));
+  return String.fromCodePoint(...codePoints);
+};