fix: adapt referral network for Telegram MiniApp safe areas

- Extract useHeaderHeight() hook — single source of truth for header
  height across AppShell, ReferralNetwork, and TicketNotificationBell
- Portal top offset now computed dynamically from TG safe area insets
  instead of hardcoded top-16 (fixes overlap with TG header bar)
- Two-row mobile layout: title + selector on separate rows so dropdown
  gets full width on small screens
- Add MOBILE_HEADER_HEIGHT_PX and DESKTOP_HEADER_HEIGHT_PX to constants
- Fix TicketNotificationBell using hardcoded 45px for all platforms

src/hooks/useHeaderHeight.ts

@@ -0,0 +1,25 @@
+import { useTelegramSDK } from '@/hooks/useTelegramSDK';
+import { UI } from '@/config/constants';
+
+/**
+ * Computes the app header height in pixels, accounting for
+ * Telegram MiniApp safe area insets in fullscreen mode.
+ *
+ * Desktop: 56px (h-14). Mobile: 64px (h-16) + safe area + TG header when fullscreen.
+ */
+export function useHeaderHeight(): { mobile: number; desktop: number } {
+  const { isFullscreen, safeAreaInset, contentSafeAreaInset, platform, isMobile } =
+    useTelegramSDK();
+  const isMobileFullscreen = isFullscreen && isMobile;
+
+  const telegramHeaderHeight =
+    platform === 'android' ? UI.TELEGRAM_HEADER_ANDROID_PX : UI.TELEGRAM_HEADER_IOS_PX;
+
+  const mobile = isMobileFullscreen
+    ? UI.MOBILE_HEADER_HEIGHT_PX +
+      Math.max(safeAreaInset.top, contentSafeAreaInset.top) +
+      telegramHeaderHeight
+    : UI.MOBILE_HEADER_HEIGHT_PX;
+
+  return { mobile, desktop: UI.DESKTOP_HEADER_HEIGHT_PX };
+}