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/lib/telegram.ts

@@ -26,8 +26,6 @@ interface TelegramWebApp {
   setBackgroundColor?: (color: string) => void;
   setBottomBarColor?: (color: string) => void;
   disableVerticalSwipes?: () => void;
-  enableClosingConfirmation?: () => void;
-  disableClosingConfirmation?: () => void;
   HapticFeedback?: {
     impactOccurred?: (style: string) => void;
     notificationOccurred?: (type: string) => void;
@@ -293,40 +291,15 @@ export function telegramHaptic(kind: Haptic): void {
 }
 
 /**
- * isMobilePlatform reports whether the Mini App runs on a Telegram mobile client — iOS or
- * Android (the latter reported as 'android' by Telegram for Android and 'android_x' by
- * Telegram X). Desktop clients (tdesktop, macOS, web) report other values. Used to limit
- * mobile-only chrome such as the close guard and immersive fullscreen.
- */
-function isMobilePlatform(): boolean {
-  const p = webApp()?.platform;
-  return p === 'ios' || p === 'android' || p === 'android_x';
-}
-
-/**
- * telegramRequestFullscreen asks Telegram to open the Mini App in fullscreen (Bot API 8.0+),
- * but only on mobile clients — mirroring how Telegram's own Mini Apps go immersive on phones
- * while staying a standard window on desktop (where the bot's full-size setting already fills
- * the window). A no-op outside Telegram, on desktop, or on clients predating the method.
+ * telegramRequestFullscreen asks Telegram to open the Mini App in immersive fullscreen (Bot API
+ * 8.0+), but only on iOS. On Android, fullscreen replaces the native header — and its BackButton —
+ * with a bare close/menu pill: the back control disappears and the Android system swipe-back falls
+ * through to minimising the app instead of navigating, so the app stays windowed there to keep the
+ * native header + BackButton (which also captures the system back). A no-op outside Telegram, on
+ * Android, on desktop, or on clients predating the method.
  */
 export function telegramRequestFullscreen(): void {
-  if (isMobilePlatform()) webApp()?.requestFullscreen?.();
-}
-
-/**
- * telegramClosingConfirmation toggles the confirmation Telegram shows when the user swipes
- * the Mini App closed — enabled during an active game so it is not lost by accident. The
- * guard is only armed on mobile clients: on desktop, closing a window is deliberate and
- * Telegram surfaces a "changes may not be saved" dialog that is just noise here (drafts
- * auto-save), so the confirmation is skipped there.
- */
-export function telegramClosingConfirmation(on: boolean): void {
-  const w = webApp();
-  if (on) {
-    if (isMobilePlatform()) w?.enableClosingConfirmation?.();
-  } else {
-    w?.disableClosingConfirmation?.();
-  }
+  if (webApp()?.platform === 'ios') webApp()?.requestFullscreen?.();
 }
 
 let backHandler: (() => void) | null = null;