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/pages/App.tsx

@@ -1,4 +1,4 @@
-import React, { ReactNode, useEffect } from 'react';
+import React, { ReactNode, useEffect, useState } from 'react';
 import { ErrorBoundary } from 'react-error-boundary';
 import { Provider as JotaiProvider, useAtomValue } from 'jotai';
 import {
@@ -23,6 +23,7 @@ import { createRouter } from './Router';
 import { ScreenSizeProvider, useScreenSize } from '../hooks/useScreenSize';
 import { useCompositionEndTracking } from '../hooks/useComposingCheck';
 import { settingsAtom } from '../state/settings';
+import { isWithinTimeWindow } from '../utils/timeWindow';
 import { LotusToastContainer } from '../features/toast/LotusToastContainer';
 import { useTauriNotificationBadge } from '../hooks/useTauriNotificationBadge';
 import { useTauriWindowChrome } from '../hooks/useTauriWindowChrome';
@@ -128,7 +129,23 @@ function DesktopChrome({ children }: { children: ReactNode }) {
 
 function NightLightOverlay() {
   const settings = useAtomValue(settingsAtom);
+  const scheduled = settings.nightLightEnabled && settings.nightLightSchedule;
+  // When scheduled, re-evaluate the active window every minute so the overlay
+  // turns on/off automatically at the start/end times without a reload.
+  const [, forceTick] = useState(0);
+  useEffect(() => {
+    if (!scheduled) return undefined;
+    const id = setInterval(() => forceTick((n) => n + 1), 60_000);
+    return () => clearInterval(id);
+  }, [scheduled]);
+
   if (!settings.nightLightEnabled) return null;
+  if (
+    settings.nightLightSchedule &&
+    !isWithinTimeWindow(settings.nightLightStart, settings.nightLightEnd)
+  ) {
+    return null;
+  }
   return (
     <div
       aria-hidden="true"