feat(ui): force Mini App fullscreen on mobile, not via the share link

Replace the shared-link &mode=fullscreen (which would also force fullscreen on
desktop) with an imperative requestFullscreen() on launch, gated to mobile
clients (ios/android/android_x) — mirroring how Telegram's own Mini Apps go
immersive on phones while desktop keeps the bot's full-size window. It triggers
the existing fullscreenChanged -> safe-area resync; a no-op on clients predating
Bot API 8.0.

ui/src/lib/telegram.ts

@@ -16,6 +16,7 @@ interface TelegramWebApp {
   contentSafeAreaInset?: { top: number; bottom: number; left: number; right: number };
   ready?: () => void;
   expand?: () => void;
+  requestFullscreen?: () => void;
   openTelegramLink?: (url: string) => void;
   onEvent?: (event: string, handler: () => void) => void;
   setHeaderColor?: (color: string) => void;
@@ -167,13 +168,24 @@ export function telegramHaptic(kind: Haptic): void {
 }
 
 /**
- * isMobilePlatform reports whether the Mini App runs on a Telegram mobile client (iOS or
- * Android), where a stray swipe-down can drop the app. Desktop clients (tdesktop, macOS,
- * web) report other values and close only on a deliberate window action.
+ * 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';
+  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.
+ */
+export function telegramRequestFullscreen(): void {
+  if (isMobilePlatform()) webApp()?.requestFullscreen?.();
 }
 
 /**