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.ts

@@ -243,6 +243,15 @@ export function telegramColorScheme(): 'light' | 'dark' | undefined {
   return webApp()?.colorScheme;
 }
 
+/**
+ * telegramThemeParams returns Telegram's current theme palette (WebApp.themeParams), or undefined
+ * outside Telegram. It reads the live value rather than a launch snapshot, so the themeChanged
+ * event can re-apply the palette when the user switches Telegram's light/dark theme mid-session.
+ */
+export function telegramThemeParams(): TelegramThemeParams | undefined {
+  return webApp()?.themeParams;
+}
+
 /**
  * telegramSetChrome paints Telegram's own header, background and bottom bar to match the
  * app's colours, so the surrounding Telegram chrome does not clash with the UI. No-op