feat(settings): sync preferences across devices via io.lotus.settings account data (#104)

Every Lotus setting was localStorage-only, so a user on web + desktop + phone
configured theme, composer toolbar, quiet hours, call keys… three times.

- utils/settingsSync.ts (pure, 7 tests): DEVICE_LOCAL_KEYS denylist (zoom,
  media auto-load, animation pause, glassmorphism, denoise tier/model,
  bitrates, volumes, notification permission, developer tools, PTT mode,
  camera-on-join, drawer state, and the sync toggle itself), pickSyncable,
  mergeRemoteSettings (unknown keys, device-local keys and wrong-shaped
  values are skipped), buildSyncedContent, shouldApplyRemote (LWW on
  updatedAt; equal stamp = our own echo).
- hooks/useSettingsSync.ts: on start applies a newer remote snapshot or
  pushes local if it differs; debounced push on any settingsAtom write,
  skipped when the syncable subset equals the last pushed/applied snapshot
  so a remote apply never echoes back; AccountData listener for live
  updates; stamps forced monotonic per device; per-account lastSyncedAt
  marker so another user on the same device can't inherit it; failed pushes
  roll the marker back so the next change retries. Remote values are re-read
  through getSettings() so enum coercion applies.
- Settings → General → Sync: toggle (device-local), "Push now", "Clear
  synced copy". AccountDataEvent.LotusSettings registered.
- ClientNonUIFeatures: the #103 tracking-param subscriber moves out of
  PageZoomFeature into its own TrackingParamsFeature next to
  SettingsSyncFeature.
- Docs: LOTUS_FEATURES entries for #103/#104; LOTUS_TODO links the new
  Features 2026-Q4 milestone and #108.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/pages/client/ClientNonUIFeatures.tsx

@@ -21,6 +21,7 @@ import { NOTIFICATION_SOUND_MAP } from '../../utils/notificationSounds';
 import { useSetting } from '../../state/hooks/settings';
 import { settingsAtom } from '../../state/settings';
 import { setStripTrackingOnRender } from '../../plugins/react-custom-html-parser';
+import { useSettingsSync } from '../../hooks/useSettingsSync';
 import { allInvitesAtom } from '../../state/room-list/inviteList';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useHydrateMsgDrafts } from '../../hooks/useHydrateMsgDrafts';
@@ -89,13 +90,23 @@ function SystemEmojiFeature() {
   return null;
 }
 
-function PageZoomFeature() {
-  const [pageZoom] = useSetting(settingsAtom, 'pageZoom');
-  // [Gitea #103] Mirror the privacy toggle into the html parser's module flag.
+// [Gitea #103] Mirror the privacy toggle into the html parser's module flag.
+function TrackingParamsFeature() {
   const [stripTracking] = useSetting(settingsAtom, 'stripTrackingParams');
   useEffect(() => {
     setStripTrackingOnRender(stripTracking);
   }, [stripTracking]);
+  return null;
+}
+
+// [Gitea #104] Mirror user preferences to account data and apply remote snapshots.
+function SettingsSyncFeature() {
+  useSettingsSync();
+  return null;
+}
+
+function PageZoomFeature() {
+  const [pageZoom] = useSetting(settingsAtom, 'pageZoom');
 
   if (pageZoom === 100) {
     document.documentElement.style.removeProperty('font-size');
@@ -893,6 +904,8 @@ export function ClientNonUIFeatures({ children }: ClientNonUIFeaturesProps) {
       <SearchCacheInvalidationFeature />
       <SystemEmojiFeature />
       <PageZoomFeature />
+      <TrackingParamsFeature />
+      <SettingsSyncFeature />
       <FaviconUpdater />
       <PresenceUpdater />
       <MuteTimerRestore />