feat(telegram): re-apply theme live on themeChanged

The Mini App read Telegram's themeParams/colorScheme only once at launch, so
switching Telegram's light/dark theme (or its auto day/night) while the app was
open left the SPA on stale colours until a relaunch.

Subscribe to the themeChanged WebApp event and re-apply the theme live. Extract
the launch-time token + colour-scheme + chrome application into syncTelegramTheme
(reading live themeParams when no launch snapshot is passed) and call it from both
applyTelegramChrome (launch) and the new event handler. Add a telegramThemeParams
live getter (+ unit test). Drop the stale 'immersive fullscreen' note from
applyTelegramChrome — the app deliberately does not request fullscreen.

ui/src/lib/telegram.test.ts

@@ -7,6 +7,7 @@ import {
   routeExternalLinkInTelegram,
   telegramLaunch,
   telegramOpenExternalLink,
+  telegramThemeParams,
 } from './telegram';
 
 function stubWebApp(initData: string, startParam?: string) {
@@ -44,6 +45,12 @@ describe('telegram launch detection', () => {
     expect(launch.startParam).toBe('g123');
     expect(launch.theme?.bg_color).toBe('#101418');
   });
+
+  it('telegramThemeParams reads the live palette (undefined outside Telegram)', () => {
+    expect(telegramThemeParams()).toBeUndefined();
+    stubWebApp('query_id=abc');
+    expect(telegramThemeParams()?.bg_color).toBe('#101418');
+  });
 });
 
 describe('telegramOpenExternalLink', () => {