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/app.svelte.ts

@@ -19,6 +19,7 @@ import {
   telegramHaptic,
   telegramLaunch,
   telegramOnEvent,
+  telegramRequestFullscreen,
   telegramSetChrome,
 } from './telegram';
 import { parseStartParam } from './deeplink';
@@ -463,6 +464,10 @@ export async function bootstrap(): Promise<void> {
     telegramOnEvent('safeAreaChanged', syncTelegramSafeArea);
     telegramOnEvent('fullscreenChanged', syncTelegramSafeArea);
     telegramDisableVerticalSwipes();
+    // On mobile, go immersive fullscreen like Telegram's own Mini Apps; the fullscreenChanged
+    // listener above then re-syncs the safe-area insets. Desktop keeps the bot's full-size
+    // window. No-op on clients predating Bot API 8.0.
+    telegramRequestFullscreen();
     try {
       await adoptSession(await gateway.authTelegram(launch.initData));
       // A blocked account skips deep-link routing — the blocked screen overlays every route.