snp-batch-validation/frontend/src/hooks/useTheme.ts

import { useState, useEffect, useCallback } from 'react';

type Theme = 'dark' | 'light';

const STORAGE_KEY = 'snp-batch-theme';

function getInitialTheme(): Theme {
  if (typeof window === 'undefined') return 'dark';
  const stored = localStorage.getItem(STORAGE_KEY);
  if (stored === 'light' || stored === 'dark') return stored;
  return 'dark';
}

export function useTheme() {
  const [theme, setTheme] = useState<Theme>(getInitialTheme);

  useEffect(() => {
    document.documentElement.setAttribute('data-theme', theme);
    localStorage.setItem(STORAGE_KEY, theme);
  }, [theme]);

  const toggle = useCallback(() => {
    setTheme((prev) => (prev === 'dark' ? 'light' : 'dark'));
  }, []);

  return { theme, toggle } as const;
}