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/telegram.ts

@@ -47,6 +47,10 @@ interface TelegramWebApp {
     onClick?: (cb: () => void) => void;
     offClick?: (cb: () => void) => void;
   };
+  CloudStorage?: {
+    getItem?: (key: string, cb: (err: string | null, value?: string) => void) => void;
+    setItem?: (key: string, value: string, cb?: (err: string | null, ok?: boolean) => void) => void;
+  };
 }
 
 function webApp(): TelegramWebApp | undefined {
@@ -312,6 +316,37 @@ export function telegramShowSettingsButton(handler: () => void): void {
   b.show();
 }
 
+/** telegramCloudAvailable reports whether Telegram CloudStorage (Bot API 6.9) is usable. */
+export function telegramCloudAvailable(): boolean {
+  return !!webApp()?.CloudStorage?.getItem;
+}
+
+/**
+ * telegramCloudGet reads a value from Telegram CloudStorage, resolving null when the key is absent,
+ * CloudStorage is unavailable (outside Telegram / a client predating Bot API 6.9), or the read
+ * errors — so the caller can fall back to the local value.
+ */
+export function telegramCloudGet(key: string): Promise<string | null> {
+  const cs = webApp()?.CloudStorage;
+  if (!cs?.getItem) return Promise.resolve(null);
+  return new Promise((resolve) => {
+    cs.getItem!(key, (err, value) => resolve(err ? null : (value ?? null)));
+  });
+}
+
+/**
+ * telegramCloudSet writes a value to Telegram CloudStorage, resolving once the write settles. It is
+ * best-effort: a no-op outside Telegram / on an older client, and it swallows write errors, since
+ * the local store remains the source of truth.
+ */
+export function telegramCloudSet(key: string, value: string): Promise<void> {
+  const cs = webApp()?.CloudStorage;
+  if (!cs?.setItem) return Promise.resolve();
+  return new Promise((resolve) => {
+    cs.setItem!(key, value, () => resolve());
+  });
+}
+
 /** Haptic is the set of feedbacks the app triggers. */
 export type Haptic = 'select' | 'success' | 'error' | 'warning' | 'light' | 'medium' | 'heavy';