Night Light was all-or-nothing. Add an optional schedule with From/To time
inputs so the warm overlay only shows during set hours and toggles itself on/off
automatically (the overlay re-checks every minute; no reload). Overnight windows
that wrap midnight (e.g. 21:00 -> 07:00) are handled.
Window logic is the pure, unit-tested isWithinTimeWindow/parseHHMM in
utils/timeWindow.ts. New settings: nightLightSchedule/Start/End (default
21:00-07:00).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
