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/app.svelte.ts

@@ -9,7 +9,7 @@ import { GatewayError } from './client';
 import { navigate, router } from './router.svelte';
 import { errorKey, localeFrom, setLocale, t, type Locale } from './i18n/index.svelte';
 import { languageNeedsServerSync } from './language';
-import { applyReduceMotion, applyTelegramTheme, applyTheme, type ThemePref } from './theme';
+import { applyReduceMotion, applyTelegramTheme, applyTheme, type ThemePref, type TelegramThemeParams } from './theme';
 import {
   insideTelegram,
   collectTelegramDiag,
@@ -18,6 +18,7 @@ import {
   hasLaunchFragment,
   loadTelegramSDK,
   telegramColorScheme,
+  telegramThemeParams,
   telegramContentSafeAreaTop,
   telegramSafeAreaTop,
   telegramDisableVerticalSwipes,
@@ -554,20 +555,31 @@ function syncViewportHeight(): void {
 }
 
 /**
- * applyTelegramChrome applies a Mini App launch's visual integration: Telegram's authoritative
- * colour scheme and theme, the matching header / background / bottom chrome, the safe-area insets,
- * the swipe-down guard, and immersive fullscreen on mobile. It is idempotent, so both the initial
- * bootstrap and a manual launch retry call it.
+ * syncTelegramTheme re-applies Telegram's theme integration — the themeParams token overrides,
+ * Telegram's authoritative colour scheme, and the matching chrome — from theme, or from the SDK's
+ * current themeParams when omitted. Called on launch with the launch snapshot and live on the
+ * themeChanged event, so switching Telegram's light/dark theme while the app is open is picked up
+ * without a relaunch.
  */
-function applyTelegramChrome(launch: TelegramLaunch): void {
-  if (launch.theme) applyTelegramTheme(launch.theme);
+function syncTelegramTheme(theme: TelegramThemeParams | undefined = telegramThemeParams()): void {
+  if (theme) applyTelegramTheme(theme);
   // Inside Telegram the colour scheme is Telegram's to decide; force it explicitly so the OS
   // prefers-color-scheme (which leaks into the Telegram Desktop webview) cannot fight it. Falls
   // back to the stored preference when the SDK omits it.
   applyTheme(telegramColorScheme() ?? app.theme);
-  // Match Telegram's chrome to the app and stop its swipe-down-to-minimise from fighting tile
-  // drag / board scroll.
   syncTelegramChrome();
+}
+
+/**
+ * applyTelegramChrome applies a Mini App launch's visual integration: Telegram's authoritative
+ * colour scheme and theme (syncTelegramTheme), the matching header / background / bottom chrome,
+ * the safe-area insets, and the swipe-down-to-minimise guard. It is idempotent, so both the
+ * initial bootstrap and a manual launch retry call it.
+ */
+function applyTelegramChrome(launch: TelegramLaunch): void {
+  syncTelegramTheme(launch.theme);
+  // Mirror the safe-area insets and stop Telegram's swipe-down-to-minimise from fighting tile drag
+  // / board scroll.
   syncTelegramSafeArea();
   telegramDisableVerticalSwipes();
 }
@@ -625,6 +637,8 @@ export async function bootstrap(): Promise<void> {
     telegramOnEvent('contentSafeAreaChanged', syncTelegramSafeArea);
     telegramOnEvent('safeAreaChanged', syncTelegramSafeArea);
     telegramOnEvent('fullscreenChanged', syncTelegramSafeArea);
+    // Re-apply the theme live when the user switches Telegram's light/dark mode while the app is open.
+    telegramOnEvent('themeChanged', () => syncTelegramTheme());
     await bootTelegram(launch);
     app.ready = true;
     return;