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/features/settings/general/General.tsx

@@ -59,6 +59,7 @@ import {
   ML_DENOISE_REQUIREMENTS,
 } from '../../../utils/lotusDenoiseUtils';
 import { useSetting } from '../../../state/hooks/settings';
+import { useSettingsSyncActions } from '../../../hooks/useSettingsSync';
 import {
   CallAudioBitrate,
   ChatBackground,
@@ -1495,6 +1496,90 @@ function Privacy() {
   );
 }
 
+// [Gitea #104] Sync toggle + the two manual actions. The toggle is itself
+// device-local (never synced) so switching it off here sticks.
+function SettingsSyncSection() {
+  const [settingsSync, setSettingsSync] = useSetting(settingsAtom, 'settingsSync');
+  const { pushNow, clearRemote } = useSettingsSyncActions();
+  const [busy, setBusy] = useState<'push' | 'clear' | null>(null);
+  const [note, setNote] = useState<string | null>(null);
+
+  const run = async (action: 'push' | 'clear') => {
+    setBusy(action);
+    setNote(null);
+    try {
+      if (action === 'push') {
+        await pushNow();
+        setNote('Pushed — other devices will pick these settings up on their next sync.');
+      } else {
+        await clearRemote();
+        setNote('Cleared — devices keep their current settings until one pushes again.');
+      }
+    } catch {
+      setNote('Failed — check your connection and try again.');
+    } finally {
+      setBusy(null);
+    }
+  };
+
+  return (
+    <Box direction="Column" gap="100">
+      <Text size="L400">Sync</Text>
+      <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+        <SettingTile
+          title="Sync Settings Across Devices"
+          description="Keep your preferences (theme, composer toolbar, notifications, quiet hours, call keys…) the same on every device, stored as account data on your own homeserver. Device-specific settings such as zoom, media auto-load, noise suppression and volumes stay local."
+          after={<Switch variant="Primary" value={settingsSync} onChange={setSettingsSync} />}
+        />
+      </SequenceCard>
+      {settingsSync && (
+        <SequenceCard className={SequenceCardStyle} variant="SurfaceVariant" direction="Column">
+          <SettingTile
+            title="Use This Device's Settings Everywhere"
+            description="Push this device's current preferences so every other device adopts them, even ones that changed something more recently."
+            after={
+              <Box gap="200">
+                <Button
+                  size="300"
+                  variant="Secondary"
+                  fill="Soft"
+                  outlined
+                  radii="300"
+                  disabled={busy !== null}
+                  before={busy === 'push' ? <Spinner size="100" variant="Secondary" /> : undefined}
+                  onClick={() => run('push')}
+                >
+                  <Text size="B300">Push now</Text>
+                </Button>
+                <Button
+                  size="300"
+                  variant="Critical"
+                  fill="None"
+                  radii="300"
+                  disabled={busy !== null}
+                  before={busy === 'clear' ? <Spinner size="100" variant="Critical" /> : undefined}
+                  onClick={() => run('clear')}
+                >
+                  <Text size="B300">Clear synced copy</Text>
+                </Button>
+              </Box>
+            }
+          />
+          {note && (
+            <Text
+              size="T200"
+              priority="300"
+              style={{ padding: `0 ${config.space.S300} ${config.space.S300}` }}
+            >
+              {note}
+            </Text>
+          )}
+        </SequenceCard>
+      )}
+    </Box>
+  );
+}
+
 // [Gitea #23] Denylist navigation-critical/modifier codes and reject a code that
 // collides with the other call key (`otherKey`), so a rebind can never trap
 // keyboard focus in-call or silently double-bind PTT and deafen to the same key.
@@ -2596,6 +2681,7 @@ export function General({ requestClose }: GeneralProps) {
               <Editor />
               <Messages />
               <Privacy />
+              <SettingsSyncSection />
               <Calls />
               <AppUpdates />
             </Box>