fix(settings): "ENTER for Newline" stays per device (#348)

Settings sync mirrored enterForNewline to io.lotus.settings, so turning it
on for a phone keyboard switched the desktop to Ctrl+Enter-to-send (and
vice versa). It is now in DEVICE_LOCAL_KEYS.

Verified with one account on a phone and a desktop profile: toggling it
on the phone leaves the desktop at Enter-to-send, and it is no longer in
the synced account data.

Fixes #348

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/settingsSync.test.ts

@@ -104,3 +104,9 @@ describe('buildSyncedContent', () => {
     assert.deepEqual(c.settings, pickSyncable(base));
   });
 });
+
+describe('device-local keys', () => {
+  it('keeps enterForNewline on each device (#348)', () => {
+    assert.equal(DEVICE_LOCAL_KEYS.has('enterForNewline'), true);
+  });
+});