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.test.ts

@@ -33,20 +33,20 @@ describe('shareLink', () => {
 
   it('wraps a payload in a startapp link', () => {
     vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/bot/app');
-    expect(shareLink('f123456')).toBe('https://t.me/bot/app?startapp=f123456&mode=fullscreen');
+    expect(shareLink('f123456')).toBe('https://t.me/bot/app?startapp=f123456');
   });
 
   it('picks the per-bot base by language', () => {
     vi.stubEnv('VITE_TELEGRAM_LINK_EN', 'https://t.me/enbot/app');
     vi.stubEnv('VITE_TELEGRAM_LINK_RU', 'https://t.me/rubot/app');
-    expect(shareLink('f1', 'en')).toBe('https://t.me/enbot/app?startapp=f1&mode=fullscreen');
-    expect(shareLink('f1', 'ru')).toBe('https://t.me/rubot/app?startapp=f1&mode=fullscreen');
+    expect(shareLink('f1', 'en')).toBe('https://t.me/enbot/app?startapp=f1');
+    expect(shareLink('f1', 'ru')).toBe('https://t.me/rubot/app?startapp=f1');
   });
 
   it('falls back to the language-agnostic base when the per-bot one is unset', () => {
     vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/fallback/app');
     vi.stubEnv('VITE_TELEGRAM_LINK_RU', '');
-    expect(shareLink('f1', 'ru')).toBe('https://t.me/fallback/app?startapp=f1&mode=fullscreen');
+    expect(shareLink('f1', 'ru')).toBe('https://t.me/fallback/app?startapp=f1');
   });
 });