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.

docs/UI_DESIGN.md

@@ -98,12 +98,12 @@ dismisses as soon as the lobby is ready. The pure layout and timing live in `lib
   theme switcher is hidden; the nav bar takes Telegram's background and `setHeaderColor` /
   `setBackgroundColor` / `setBottomBarColor` paint Telegram's own chrome to match; the
   native header **BackButton** drives back-navigation (the app's chevron is hidden in
-  Telegram); **HapticFeedback** fires on tile placement / commit / error; on **mobile**
-  clients the app enters **immersive fullscreen** on launch (`requestFullscreen`, Bot API
-  8.0+) like Telegram's own Mini Apps, while desktop keeps the bot's full-size window;
-  **closing confirmation** is enabled while a game is open **on mobile only** (on desktop
-  closing is deliberate and the "changes may not be saved" dialog is just noise — move drafts
-  auto-save); **vertical swipes** (swipe-to-minimise)
+  Telegram); **HapticFeedback** fires on tile placement / commit / error; on **iOS** the app
+  enters **immersive fullscreen** on launch (`requestFullscreen`, Bot API 8.0+) like Telegram's
+  own Mini Apps, while **Android stays windowed** — there fullscreen hides the native header and
+  its BackButton, and the Android system swipe-back then minimises the app instead of navigating —
+  and desktop keeps the bot's full-size window; move drafts auto-save, so there is **no
+  closing-confirmation guard**; **vertical swipes** (swipe-to-minimise)
   are disabled so they don't fight tile drag or the board scroll; **external links** (the word-check
   dictionary lookup, the rules link, operator-reply links) open through `Telegram.WebApp.openLink`
   so Telegram shows them in its in-app browser instead of the WebView's "open this link?"