Add useDarkMode hook

src/hooks/useDarkMode.ts

@@ -0,0 +1,15 @@
+import { useState, useEffect } from "react";
+
+export function useDarkMode() {
+  const [enabled, setEnabled] = useState(() => {
+    if (typeof window === "undefined") return false;
+    return localStorage.getItem("theme") === "dark";
+  });
+
+  useEffect(() => {
+    document.documentElement.classList.toggle("dark", enabled);
+    localStorage.setItem("theme", enabled ? "dark" : "light");
+  }, [enabled]);
+
+  return { enabled, toggle: () => setEnabled((v) => !v) };
+}