feat: tint app background by time of day (F-5)

Adds dawn/day/dusk/night palette derived from device-local hour. Root
background fades over 1.5s on transitions; recalculated every 5 minutes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -15,6 +15,7 @@ import { TutorialOverlay } from './components/tutorial/TutorialOverlay';
 import { useIdleTick } from './hooks/useIdleTick';
 import { useGameStore } from './store/useGameStore';
 import { trackGameEvent } from './platform/analytics';
+import { useTimeOfDay, getPalette } from './lib/timeOfDay';
 
 const legendaryShake = keyframes`
   0%, 100% { transform: translate3d(0, 0, 0); filter: none; }
@@ -25,12 +26,12 @@ const legendaryShake = keyframes`
   75% { transform: translate3d(-2px, 1px, 0); }
 `;
 
-const rootStyle = (shaking: boolean) => css`
+const rootStyle = (shaking: boolean, background: string) => css`
   width: 100%;
   height: 100vh;
   display: flex;
   flex-direction: column;
-  background-color: #f7f8fa;
+  background-color: ${background};
   overflow: hidden;
   font-family:
     'Pretendard',
@@ -38,6 +39,7 @@ const rootStyle = (shaking: boolean) => css`
     BlinkMacSystemFont,
     sans-serif;
   animation: ${shaking ? css`${legendaryShake} 0.42s ease-out` : 'none'};
+  transition: background-color 1.5s ease-in-out;
 `;
 
 const contentStyle = css`
@@ -51,6 +53,8 @@ export function App() {
   const [shaking, setShaking] = useState(false);
   const [showIntro, setShowIntro] = useState(() => localStorage.getItem('firstspark-intro-seen') !== '1');
   useIdleTick();
+  const timeOfDay = useTimeOfDay();
+  const palette = getPalette(timeOfDay);
 
   const handleIntroDone = useCallback(() => {
     localStorage.setItem('firstspark-intro-seen', '1');
@@ -79,7 +83,7 @@ export function App() {
   }, []);
 
   return (
-    <div css={rootStyle(shaking)}>
+    <div css={rootStyle(shaking, palette.background)}>
       {showIntro && (
         <IntroSplash
           onDone={handleIntroDone}

src/lib/timeOfDay.ts

@@ -0,0 +1,43 @@
+import { useEffect, useState } from 'react';
+
+export type TimeOfDay = 'dawn' | 'day' | 'dusk' | 'night';
+
+export interface TimeOfDayPalette {
+  background: string;
+  contentTint: string;
+}
+
+const PALETTES: Record<TimeOfDay, TimeOfDayPalette> = {
+  dawn:  { background: '#fff6ec', contentTint: 'rgba(255, 196, 138, 0.05)' },
+  day:   { background: '#f7f8fa', contentTint: 'rgba(255, 255, 255, 0)' },
+  dusk:  { background: '#fff0e6', contentTint: 'rgba(255, 138, 92, 0.06)' },
+  night: { background: '#101728', contentTint: 'rgba(124, 138, 255, 0.04)' },
+};
+
+export function getTimeOfDay(date: Date = new Date()): TimeOfDay {
+  const h = date.getHours();
+  if (h >= 5 && h < 9) return 'dawn';
+  if (h >= 9 && h < 17) return 'day';
+  if (h >= 17 && h < 20) return 'dusk';
+  return 'night';
+}
+
+export function getPalette(tod: TimeOfDay): TimeOfDayPalette {
+  return PALETTES[tod];
+}
+
+const TICK_MS = 5 * 60 * 1000;
+
+export function useTimeOfDay(): TimeOfDay {
+  const [tod, setTod] = useState<TimeOfDay>(() => getTimeOfDay());
+  useEffect(() => {
+    const id = window.setInterval(() => {
+      setTod((prev) => {
+        const next = getTimeOfDay();
+        return next === prev ? prev : next;
+      });
+    }, TICK_MS);
+    return () => window.clearInterval(id);
+  }, []);
+  return tod;
+}