frontend/src/lib/theme.tsx

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

type Theme = "light" | "dark";

interface ThemeCtx {
  theme: Theme;
  toggle: () => void;
}

const Ctx = createContext<ThemeCtx | null>(null);

function initialTheme(): Theme {
  const stored = localStorage.getItem("theme");
  if (stored === "light" || stored === "dark") return stored;
  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
}

export function ThemeProvider({ children }: { children: ReactNode }) {
  const [theme, setTheme] = useState<Theme>(initialTheme);

  useEffect(() => {
    const root = document.documentElement;
    root.classList.toggle("dark", theme === "dark");
    localStorage.setItem("theme", theme);
  }, [theme]);

  const toggle = () => setTheme((t) => (t === "dark" ? "light" : "dark"));

  return <Ctx.Provider value={{ theme, toggle }}>{children}</Ctx.Provider>;
}

export function useTheme() {
  const ctx = useContext(Ctx);
  if (!ctx) throw new Error("useTheme must be used within ThemeProvider");
  return ctx;
}