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.ts

@@ -0,0 +1,49 @@
+// Telegram CloudStorage sync for the device-independent client display preferences — theme,
+// reduce-motion and board labels — so they follow the user across their Telegram devices. The
+// interface language is intentionally excluded: it has its own server-side sync
+// (Profile.preferredLanguage) plus an on-launch reconciler, and mixing it in here would fight that.
+// The pure encode/decode is kept free of the SDK and the DOM so it unit-tests in the node
+// environment; the CloudStorage transport wrappers live in telegram.ts and the wiring (mirror on
+// save, reconcile on launch) in app.svelte.ts.
+
+import type { ThemePref } from './theme';
+import type { BoardLabelMode } from './boardlabels';
+
+/** ClientPrefs is the subset of preferences synced across devices via Telegram CloudStorage. */
+export interface ClientPrefs {
+  theme: ThemePref;
+  reduceMotion: boolean;
+  boardLabels: BoardLabelMode;
+}
+
+/** CLOUD_PREFS_KEY is the Telegram CloudStorage key holding the JSON-encoded ClientPrefs. */
+export const CLOUD_PREFS_KEY = 'prefs';
+
+/** encodeClientPrefs serialises the synced client prefs (and only those — never the locale). */
+export function encodeClientPrefs(p: ClientPrefs): string {
+  return JSON.stringify({ theme: p.theme, reduceMotion: p.reduceMotion, boardLabels: p.boardLabels });
+}
+
+/**
+ * decodeClientPrefs parses a CloudStorage payload into a partial ClientPrefs, keeping only valid
+ * fields and dropping anything unknown, mistyped or malformed — so a value written by a newer or
+ * older build, or a corrupt entry, never throws and never applies a bad setting. A missing field
+ * stays absent, so the caller leaves the corresponding local value untouched.
+ */
+export function decodeClientPrefs(raw: string | null | undefined): Partial<ClientPrefs> {
+  if (!raw) return {};
+  let o: Record<string, unknown>;
+  try {
+    o = JSON.parse(raw) as Record<string, unknown>;
+  } catch {
+    return {};
+  }
+  if (!o || typeof o !== 'object') return {};
+  const out: Partial<ClientPrefs> = {};
+  if (o.theme === 'auto' || o.theme === 'light' || o.theme === 'dark') out.theme = o.theme;
+  if (typeof o.reduceMotion === 'boolean') out.reduceMotion = o.reduceMotion;
+  if (o.boardLabels === 'beginner' || o.boardLabels === 'classic' || o.boardLabels === 'none') {
+    out.boardLabels = o.boardLabels;
+  }
+  return out;
+}