feat(ui): refine Telegram invite & close UX

- Shared invite links open the Mini App fullscreen (mode=fullscreen), so a
  shared link matches the bot's own fullscreen launch.
- A used or expired invite deep-link now lands the visitor in the lobby with a
  gentle notice pointing at the right bot (@<username>, by service language),
  instead of a red "code invalid/expired" error on the Friends screen.
- The in-game close confirmation is armed only on Telegram mobile clients; on
  desktop (tdesktop/macOS/web) it is skipped, where the "changes may not be
  saved" dialog is just noise (drafts auto-save).

ui/src/lib/deeplink.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { friendCodeParam, gameParam, invitationParam, parseStartParam, shareLink } from './deeplink';
+import { botUsername, friendCodeParam, gameParam, invitationParam, parseStartParam, shareLink } from './deeplink';
 
 describe('parseStartParam', () => {
   it('classifies game / invitation / friend code', () => {
@@ -33,19 +33,40 @@ 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');
+    expect(shareLink('f123456')).toBe('https://t.me/bot/app?startapp=f123456&mode=fullscreen');
   });
 
   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');
-    expect(shareLink('f1', 'ru')).toBe('https://t.me/rubot/app?startapp=f1');
+    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');
   });
 
   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');
+    expect(shareLink('f1', 'ru')).toBe('https://t.me/fallback/app?startapp=f1&mode=fullscreen');
+  });
+});
+
+describe('botUsername', () => {
+  afterEach(() => vi.unstubAllEnvs());
+
+  it('returns null without a configured base', () => {
+    vi.stubEnv('VITE_TELEGRAM_LINK', '');
+    expect(botUsername()).toBeNull();
+  });
+
+  it('extracts the bot handle from the Mini App link', () => {
+    vi.stubEnv('VITE_TELEGRAM_LINK', 'https://t.me/bot/app');
+    expect(botUsername()).toBe('bot');
+  });
+
+  it('picks the per-bot handle 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(botUsername('en')).toBe('enbot');
+    expect(botUsername('ru')).toBe('rubot');
   });
 });