feat(a11y): cross-platform hardening + modal focus-trap from impeccable audit

Responsive / viewport:
- add .min-h-viewport / .h-viewport utilities (100dvh + Telegram --tg-viewport-stable-height)
- migrate min-h-screen / h-screen / 100vh across 18 files
- reveal hover-only controls on focus-within + touch (desktop nav, admin settings)
- grid breakpoints (TopUpAmount, Contests), larger touch targets, nav aria-labels

Platform routing (Telegram WebView correctness):
- clipboard -> copyToClipboard util with execCommand fallback (10 files)
- window.open -> openTelegramLink / openLink (Profile, Referral, ChannelSubscriptionScreen)
- window.confirm -> useNativeDialog (admin pages); PromoOffersSection -> useDestructiveConfirm
- window.prompt -> usePrompt / PromptDialogHost (TipTap link editors)
- ESLint no-restricted-properties guard against regressions (adapters/clipboard util exempt)

Modal accessibility:
- new useFocusTrap hook (focus trap, Esc, scroll lock, focus restore)
- role=dialog / aria-modal / focus-trap: Polls, SuccessNotificationModal, AdminPolicies,
  AdminPromoGroups, AdminCampaigns, BroadcastPreview (Telegram + Email)
- new global usePrompt store + PromptDialogHost

src/pages/AdminPromocodes.tsx

@@ -5,6 +5,7 @@ import { useTranslation } from 'react-i18next';
 import i18n from '../i18n';
 import { promocodesApi, PromoCode, PromoCodeType } from '../api/promocodes';
 import { usePlatform } from '../platform/hooks/usePlatform';
+import { copyToClipboard } from '../utils/clipboard';
 
 // Icons
 
@@ -131,7 +132,7 @@ export default function AdminPromocodes() {
   });
 
   const handleCopyCode = (code: string) => {
-    navigator.clipboard.writeText(code);
+    void copyToClipboard(code);
     setCopiedCode(code);
     setTimeout(() => setCopiedCode(null), 2000);
   };