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

eslint.config.js

@@ -39,6 +39,49 @@ export default tseslint.config(
       'no-implied-eval': 'error',
       'no-new-func': 'error',
       'no-script-url': 'error',
+      // Cross-platform guard: these browser APIs misbehave inside the Telegram WebView.
+      // Route them through the platform abstraction instead. The platform adapters and
+      // the clipboard util (the canonical implementations) are exempted below.
+      'no-restricted-properties': [
+        'error',
+        {
+          object: 'window',
+          property: 'confirm',
+          message:
+            'Use useNativeDialog().confirm — window.confirm is silently ignored in the Telegram WebView.',
+        },
+        {
+          object: 'window',
+          property: 'alert',
+          message:
+            'Use useNativeDialog().alert — window.alert is silently ignored in the Telegram WebView.',
+        },
+        {
+          object: 'window',
+          property: 'open',
+          message:
+            'Use usePlatform().openLink / openTelegramLink — window.open is intercepted by the Telegram WebView.',
+        },
+        {
+          object: 'window',
+          property: 'prompt',
+          message:
+            'Use usePrompt() (PromptDialogHost) — window.prompt is not supported in the Telegram WebView.',
+        },
+        {
+          object: 'navigator',
+          property: 'clipboard',
+          message:
+            'Use copyToClipboard from @/utils/clipboard — it falls back to execCommand when the Clipboard API is unavailable (Telegram WebView).',
+        },
+      ],
+    },
+  },
+  {
+    // Canonical implementations that intentionally call the restricted browser APIs.
+    files: ['src/platform/**/*.{ts,tsx}', 'src/utils/clipboard.ts'],
+    rules: {
+      'no-restricted-properties': 'off',
     },
   },
   eslintConfigPrettier,