feat(ui): refine Telegram invite & close UX

- Shared invite links open the Mini App fullscreen (mode=fullscreen), so a
  shared link matches the bot's own fullscreen launch.
- A used or expired invite deep-link now lands the visitor in the lobby with a
  gentle notice pointing at the right bot (@<username>, by service language),
  instead of a red "code invalid/expired" error on the Friends screen.
- The in-game close confirmation is armed only on Telegram mobile clients; on
  desktop (tdesktop/macOS/web) it is skipped, where the "changes may not be
  saved" dialog is just noise (drafts auto-save).

ui/src/App.svelte

@@ -6,6 +6,7 @@
   import { t } from './lib/i18n/index.svelte';
   import { insideTelegram, telegramBackButton } from './lib/telegram';
   import Toast from './components/Toast.svelte';
+  import StaleInviteModal from './components/StaleInviteModal.svelte';
   import Login from './screens/Login.svelte';
   import Lobby from './screens/Lobby.svelte';
   import NewGame from './screens/NewGame.svelte';
@@ -108,6 +109,7 @@
 {/if}
 
 <Toast />
+<StaleInviteModal />
 
 <style>
   .splash {