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/components/Header.svelte

@@ -1,14 +1,19 @@
 <script lang="ts">
   import type { Snippet } from 'svelte';
   import { navigate } from '../lib/router.svelte';
+  import { insideTelegram } from '../lib/telegram';
 
   let { title, back, menu, grow = false }: { title: string; back?: string; menu?: Snippet; grow?: boolean } =
     $props();
+
+  // Inside Telegram the native header back button (App.svelte) is the back control, so
+  // the app's own chevron is hidden to avoid two back affordances.
+  const showBack = $derived(!!back && !insideTelegram());
 </script>
 
 <header class="nav" class:grow>
   <div class="bar">
-    {#if back}
+    {#if showBack}
       <button class="icon back" onclick={() => back && navigate(back)} aria-label="Back">
         <span class="chev"></span>
       </button>