Stage 17 (contour round 3): Telegram Mini Apps polish, board scroll, keyboard overlay

- Telegram (lib/telegram.ts): chrome colours (setHeaderColor/setBackgroundColor/setBottomBarColor) match Telegram's header/bg/bottom bar to the app; native BackButton on sub-screens (app chevron hidden in TG); HapticFeedback on tile place/commit/error; enableClosingConfirmation while a game is open; disableVerticalSwipes so swipe-to-minimise doesn't fight tile drag / board scroll
- #9 board-only vertical scroll: Screen 'column' mode lets the board area scroll while score/status/rack/tab bar stay fixed (zoom keeps its own scroll)
- #10 check-word dialog opens in Modal keyboard-overlay mode (top-anchored, keyboard overlays the empty area) — no resize/relayout jank; other modals stay keyboard-aware
- docs: UI_DESIGN Telegram integration + vertical fit/keyboard; PLAN round 2-3 follow-ups

ui/src/App.svelte

@@ -2,8 +2,9 @@
   import { onMount } from 'svelte';
   import { cubicOut } from 'svelte/easing';
   import { app, bootstrap } from './lib/app.svelte';
-  import { router } from './lib/router.svelte';
+  import { navigate, router } from './lib/router.svelte';
   import { t } from './lib/i18n/index.svelte';
+  import { insideTelegram, telegramBackButton } from './lib/telegram';
   import Toast from './components/Toast.svelte';
   import Login from './screens/Login.svelte';
   import Lobby from './screens/Lobby.svelte';
@@ -19,6 +20,15 @@
     void bootstrap();
   });
 
+  // Inside Telegram, drive its native header back button: show it on any sub-screen
+  // (everything returns to the lobby root), hide it on the lobby/login. The app's own
+  // back chevron is hidden in Telegram (Header.svelte) so only the native one shows.
+  $effect(() => {
+    if (!insideTelegram()) return;
+    const name = router.route.name;
+    telegramBackButton(name !== 'lobby' && name !== 'login', () => navigate('/'));
+  });
+
   // Screen transitions: the lobby is the navigation root. Entering a screen from the
   // lobby slides it in from the right (forward); returning to the lobby slides the
   // screen out to the right and reveals the lobby (back). Transitions are local, so