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/telegram.test.ts

@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from 'vitest';
-import { insideTelegram, telegramLaunch } from './telegram';
+import { insideTelegram, telegramClosingConfirmation, telegramLaunch } from './telegram';
 
 function stubWebApp(initData: string, startParam?: string) {
   vi.stubGlobal('window', {
@@ -37,3 +37,40 @@ describe('telegram launch detection', () => {
     expect(launch.theme?.bg_color).toBe('#101418');
   });
 });
+
+describe('telegramClosingConfirmation', () => {
+  afterEach(() => vi.unstubAllGlobals());
+
+  // stubClient stands up a fake WebApp on the given platform with spies for the close-guard
+  // toggles, so the platform gate can be asserted without a real Telegram client.
+  function stubClient(platform?: string) {
+    const enable = vi.fn();
+    const disable = vi.fn();
+    vi.stubGlobal('window', {
+      Telegram: { WebApp: { platform, enableClosingConfirmation: enable, disableClosingConfirmation: disable } },
+    });
+    return { enable, disable };
+  }
+
+  it('arms the close guard on mobile clients', () => {
+    for (const p of ['ios', 'android']) {
+      const { enable } = stubClient(p);
+      telegramClosingConfirmation(true);
+      expect(enable, `platform=${p}`).toHaveBeenCalledOnce();
+    }
+  });
+
+  it('skips the close guard on desktop clients (the dialog there is just noise)', () => {
+    for (const p of ['tdesktop', 'macos', 'web', undefined]) {
+      const { enable } = stubClient(p);
+      telegramClosingConfirmation(true);
+      expect(enable, `platform=${p}`).not.toHaveBeenCalled();
+    }
+  });
+
+  it('always lifts the guard on leave, regardless of platform', () => {
+    const { disable } = stubClient('tdesktop');
+    telegramClosingConfirmation(false);
+    expect(disable).toHaveBeenCalledOnce();
+  });
+});