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/styles/globals.css

@@ -1111,6 +1111,18 @@ img.twemoji {
 }
 
 @layer utilities {
+  /* Viewport height — correct across web (dvh) and Telegram Mini App.
+     main.tsx binds --tg-viewport-stable-height via bindViewportCssVars();
+     on the standalone web build the var is undefined and falls back to 100dvh. */
+  .min-h-viewport {
+    min-height: 100dvh;
+    min-height: var(--tg-viewport-stable-height, 100dvh);
+  }
+  .h-viewport {
+    height: 100dvh;
+    height: var(--tg-viewport-stable-height, 100dvh);
+  }
+
   /* Text gradient */
   .text-gradient {
     @apply bg-gradient-to-r from-accent-400 to-accent-600 bg-clip-text text-transparent;