Adds POST /api/system/time-sync so the dashboard's "Sync with this browser" button can set the host's date, time, and IANA timezone from the client's browser — useful for appliances with no NTP/network access to keep their own clock straight. Disables NTP before writing the manual time so it doesn't get immediately overwritten. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>