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/cloudprefs.test.ts

@@ -0,0 +1,31 @@
+import { describe, expect, it } from 'vitest';
+import { CLOUD_PREFS_KEY, decodeClientPrefs, encodeClientPrefs } from './cloudprefs';
+
+describe('cloudprefs', () => {
+  it('round-trips the synced client prefs', () => {
+    const p = { theme: 'dark', reduceMotion: true, boardLabels: 'classic' } as const;
+    expect(decodeClientPrefs(encodeClientPrefs(p))).toEqual(p);
+  });
+
+  it('never encodes the locale (it syncs via the durable account instead)', () => {
+    const raw = encodeClientPrefs({ theme: 'light', reduceMotion: false, boardLabels: 'none' });
+    expect(raw).not.toContain('locale');
+  });
+
+  it('returns an empty partial for missing or malformed input', () => {
+    expect(decodeClientPrefs(null)).toEqual({});
+    expect(decodeClientPrefs(undefined)).toEqual({});
+    expect(decodeClientPrefs('')).toEqual({});
+    expect(decodeClientPrefs('not json')).toEqual({});
+    expect(decodeClientPrefs('[1,2,3]')).toEqual({});
+  });
+
+  it('keeps only valid fields and drops unknown or mistyped ones', () => {
+    const raw = JSON.stringify({ theme: 'neon', reduceMotion: 'yes', boardLabels: 'classic', locale: 'ru' });
+    expect(decodeClientPrefs(raw)).toEqual({ boardLabels: 'classic' });
+  });
+
+  it('exposes the CloudStorage key', () => {
+    expect(CLOUD_PREFS_KEY).toBe('prefs');
+  });
+});