refactor: reorganize project structure — frontend/, infra/, clean paths

frontend/src/hooks/useTheme.ts

@@ -0,0 +1,20 @@
+import { useState, useCallback, useEffect } from 'react';
+
+type Theme = 'light' | 'dark';
+
+export function useTheme() {
+  const [theme, setTheme] = useState<Theme>(() => {
+    return (localStorage.getItem('theme') as Theme) || 'light';
+  });
+
+  useEffect(() => {
+    document.documentElement.className = theme;
+    localStorage.setItem('theme', theme);
+  }, [theme]);
+
+  const toggle = useCallback(() => {
+    setTheme((t) => (t === 'dark' ? 'light' : 'dark'));
+  }, []);
+
+  return { theme, toggle, isDark: theme === 'dark' };
+}