INSIGHT-MVP/packages/frontend/src/theme/ThemeContext.tsx

import {
  createContext,
  useContext,
  useState,
  useEffect,
  type ReactNode,
} from 'react';

type ThemeMode = 'light' | 'dark' | 'system';

interface ThemeContextType {
  mode: ThemeMode;
  setMode: (mode: ThemeMode) => void;
  /** Tatsaechlich aktives Theme (resolved from system) */
  resolved: 'light' | 'dark';
}

const ThemeContext = createContext<ThemeContextType>({
  mode: 'system',
  setMode: () => {},
  resolved: 'light',
});

export function useTheme() {
  return useContext(ThemeContext);
}

function getSystemTheme(): 'light' | 'dark' {
  if (
    typeof window !== 'undefined' &&
    window.matchMedia('(prefers-color-scheme: dark)').matches
  ) {
    return 'dark';
  }
  return 'light';
}

export function ThemeProvider({ children }: { children: ReactNode }) {
  const [mode, setModeState] = useState<ThemeMode>(() => {
    const saved = localStorage.getItem('theme-mode');
    if (saved === 'light' || saved === 'dark' || saved === 'system') {
      return saved;
    }
    return 'system';
  });

  const [systemTheme, setSystemTheme] = useState<'light' | 'dark'>(
    getSystemTheme,
  );

  // Listen for system theme changes
  useEffect(() => {
    const mq = window.matchMedia('(prefers-color-scheme: dark)');
    const handler = (e: MediaQueryListEvent) => {
      setSystemTheme(e.matches ? 'dark' : 'light');
    };
    mq.addEventListener('change', handler);
    return () => mq.removeEventListener('change', handler);
  }, []);

  const resolved = mode === 'system' ? systemTheme : mode;

  // Apply theme to document
  useEffect(() => {
    document.documentElement.setAttribute('data-theme', resolved);
  }, [resolved]);

  const setMode = (newMode: ThemeMode) => {
    setModeState(newMode);
    localStorage.setItem('theme-mode', newMode);
  };

  return (
    <ThemeContext.Provider value={{ mode, setMode, resolved }}>
      {children}
    </ThemeContext.Provider>
  );
}