Add Header.js

frontend/src/Header.js

+import {Link} from "react-router-dom";
+import logo from './logo.png';
+
+function Header(){
+    return(
+        <div className="header">
+            <Link to='/'><img className="logo_image" alt="logo" src={logo}/></Link>
+            <ul className="menu_list">
+                <li><Link to="/">Home</Link></li>
+                <li><Link to="/search">검색</Link></li>
+                <li><Link to="/postwrite">포스트 작성</Link></li>
+                <li><Link to="/login">로그인</Link></li> {/*로그인 여부 로직 구현 필요*/}
+                {/* { Object.keys(user).length != 0 ? 
+                    <li><Link to={`/profile/${getUserId()}`}>profile</Link>/<span onClick={logout}>logout</span></li> :
+                    <li><Link to="/login">login</Link></li>
+                }   */}
+            </ul>
+        </div>
+    );
+}
+
+export default Header;