refactor: Move useOutsideClick hook to general hooks directory

- Move useOutsideClick hook from scaffold-eth specific directory to general hooks
- Update import path in Header component
- Part of UI consistency updates

hooks/useOutsideClick.ts

@@ -0,0 +1,23 @@
+import React, { useEffect } from "react";
+
+/**
+ * Handles clicks outside of passed ref element
+ * @param ref - react ref of the element
+ * @param callback - callback function to call when clicked outside
+ */
+export const useOutsideClick = (ref: React.RefObject<HTMLElement>, callback: { (): void }) => {
+  useEffect(() => {
+    function handleOutsideClick(event: MouseEvent) {
+      if (!(event.target instanceof Element)) {
+        return;
+      }
+
+      if (ref.current && !ref.current.contains(event.target)) {
+        callback();
+      }
+    }
+
+    document.addEventListener("click", handleOutsideClick);
+    return () => document.removeEventListener("click", handleOutsideClick);
+  }, [ref, callback]);
+};