feat(night-light): optional auto-on schedule (time window)

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>

src/app/state/settings.ts

@@ -220,6 +220,9 @@ export interface Settings {
 
   nightLightEnabled: boolean;
   nightLightOpacity: number;
+  nightLightSchedule: boolean;
+  nightLightStart: string;
+  nightLightEnd: string;
 
   glassmorphismSidebar: boolean;
 
@@ -329,6 +332,9 @@ const defaultSettings: Settings = {
   pttMode: false,
   pttKey: 'Space',
 
+  nightLightSchedule: false,
+  nightLightStart: '21:00',
+  nightLightEnd: '07:00',
   nightLightEnabled: false,
   nightLightOpacity: 30,