feat(telegram): sync client display prefs via CloudStorage

Theme, reduce-motion and board labels lived only in local IndexedDB/localStorage,
so they did not follow the user across devices and could be lost when the Telegram
WebView cleared storage.

Mirror these three device-independent prefs to Telegram CloudStorage (Bot API 6.9)
from the single local-persist point (persistPrefs), and reconcile them from
CloudStorage in the background on launch (reconcileCloudPrefs) so a change made on
another device follows the user here. The local store stays the instant-render
cache; the interface language is intentionally excluded (it syncs via the durable
account). Pure encode/decode extracted to cloudprefs.ts with unit tests; the
CloudStorage transport wrappers are added to telegram.ts. No-op outside Telegram or
on a client predating CloudStorage.

ui/src/lib/app.svelte.ts

@@ -28,7 +28,11 @@ import {
   type TelegramLaunch,
   telegramOnEvent,
   telegramSetChrome,
+  telegramCloudAvailable,
+  telegramCloudGet,
+  telegramCloudSet,
 } from './telegram';
+import { CLOUD_PREFS_KEY, decodeClientPrefs, encodeClientPrefs } from './cloudprefs';
 import { parseStartParam } from './deeplink';
 import { clearSession, loadPrefs, loadSession, saveSession, savePrefs } from './session';
 import { connection, reportOffline, reportOnline, resetConnection } from './connection.svelte';
@@ -642,6 +646,10 @@ export async function bootstrap(): Promise<void> {
   if (insideTelegram()) {
     const launch = telegramLaunch();
     applyTelegramChrome(launch);
+    // Pull the device-independent display prefs (theme / reduce-motion / board labels) from
+    // CloudStorage in the background so a change on another device follows the user here; the local
+    // values applied above render instantly, so this reconciles without blocking launch.
+    void reconcileCloudPrefs();
     // Re-sync the safe-area insets whenever Telegram's chrome changes (registered once per load).
     telegramOnEvent('contentSafeAreaChanged', syncTelegramSafeArea);
     telegramOnEvent('safeAreaChanged', syncTelegramSafeArea);
@@ -837,6 +845,42 @@ function persistPrefs(): void {
     reduceMotion: app.reduceMotion,
     boardLabels: app.boardLabels,
   });
+  // Mirror the device-independent display prefs to Telegram CloudStorage so they follow the user
+  // across devices (no-op outside Telegram / on a client predating it). Locale is excluded — it
+  // syncs via the durable account (Profile.preferredLanguage) instead.
+  void telegramCloudSet(
+    CLOUD_PREFS_KEY,
+    encodeClientPrefs({ theme: app.theme, reduceMotion: app.reduceMotion, boardLabels: app.boardLabels }),
+  );
+}
+
+/**
+ * reconcileCloudPrefs pulls the device-independent display prefs (theme / reduce-motion / board
+ * labels) from Telegram CloudStorage and applies any that differ from the current values, so a
+ * change made on another Telegram device follows the user here. The local store is the
+ * instant-render cache (read synchronously at boot); this runs once on launch after it and persists
+ * what it applied. Theme is not re-applied visually — inside Telegram the colour scheme is
+ * Telegram's to decide — only its stored value is updated. A no-op outside Telegram or when
+ * CloudStorage is unavailable; locale is never synced this way (it has its own server reconciler).
+ */
+async function reconcileCloudPrefs(): Promise<void> {
+  if (!telegramCloudAvailable()) return;
+  const cloud = decodeClientPrefs(await telegramCloudGet(CLOUD_PREFS_KEY));
+  let changed = false;
+  if (cloud.theme !== undefined && cloud.theme !== app.theme) {
+    app.theme = cloud.theme;
+    changed = true;
+  }
+  if (cloud.reduceMotion !== undefined && cloud.reduceMotion !== app.reduceMotion) {
+    app.reduceMotion = cloud.reduceMotion;
+    applyReduceMotion(app.reduceMotion);
+    changed = true;
+  }
+  if (cloud.boardLabels !== undefined && cloud.boardLabels !== app.boardLabels) {
+    app.boardLabels = cloud.boardLabels;
+    changed = true;
+  }
+  if (changed) persistPrefs();
 }
 
 export function setTheme(theme: ThemePref): void {