fix(ui): Android Telegram nav — windowed mode, no close guard

Three Android Mini App issues, all in the Telegram chrome:

- Entering a game showed no native back button, and the Android system
  swipe-back minimised the app instead of navigating. Root cause: immersive
  fullscreen (requestFullscreen). On Android, fullscreen replaces the native
  header — and its BackButton, which also captures the system back — with a bare
  close/menu pill, so back navigation has no control to land on. Request
  fullscreen on iOS only; Android stays windowed, keeping the native header +
  BackButton (and the system swipe-back that routes to it). iOS is unchanged.

- Closing the game board always prompted "changes that you made may not be
  saved", even on a board just opened and untouched. The close-confirmation was
  armed unconditionally on game mount. Remove it entirely: move drafts auto-save
  (debounced during play + flushed on destroy), so nothing is lost on close.

Drops the now-unused telegramClosingConfirmation + isMobilePlatform helpers and
their SDK interface fields. Docs: UI_DESIGN.md.

ui/src/game/Game.svelte

@@ -24,7 +24,7 @@
   import { getCachedGame, setCachedGame, setCachedDraft, type CachedGame } from '../lib/gamecache';
   import { patchLobbyGame } from '../lib/lobbycache';
   import { applyGameOver, applyMoveDelta, applyOpponentJoined, type DeltaResult } from '../lib/gamedelta';
-  import { telegramClosingConfirmation, telegramHaptic } from '../lib/telegram';
+  import { telegramHaptic } from '../lib/telegram';
   import {
     BLANK,
     newPlacement,
@@ -228,8 +228,6 @@
     placement = tiles.length ? placementFromHint(tiles, rack) : newPlacement(rack);
   }
   onMount(() => {
-    // Guard against an accidental swipe-close losing the open game (Telegram).
-    telegramClosingConfirmation(true);
     // Render instantly from the cache (a game opened before), then refresh in the
     // background. A cold open shows the loading state until load() resolves.
     const cached = getCachedGame(id);
@@ -579,7 +577,6 @@
       clearTimeout(draftSaveTimer);
       void gateway.draftSave(id, serializeDraft(rackIds, placement.pending)).catch(() => {});
     }
-    telegramClosingConfirmation(false);
   });
 
   function onCell(row: number, col: number) {