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/deeplink.ts

@@ -78,7 +78,5 @@ export function shareLink(param: string, lang = ''): string | null {
   const base = telegramBase(lang);
   if (!base) return null;
   const sep = base.includes('?') ? '&' : '?';
-  // mode=fullscreen opens the Mini App fullscreen, matching how the bot's own entry
-  // opens it, so a shared link and the bot give the same experience.
-  return `${base}${sep}startapp=${encodeURIComponent(param)}&mode=fullscreen`;
+  return `${base}${sep}startapp=${encodeURIComponent(param)}`;
 }