dashboard 형태의 UI 수정 및 고도화

src/App.jsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import { Outlet } from 'react-router-dom';
 import Navbar from './components/Navbar';
+import PageHeader from './components/PageHeader';
 import Loading from './components/Loading';
 import './App.css';
 
@@ -10,6 +11,7 @@ function App() {
             <Navbar />
             <div className="app-content">
                 <main className="site-main">
+                    <PageHeader />
                     <React.Suspense fallback={<div className="suspend-loading"><Loading /></div>}>
                         <Outlet />
                     </React.Suspense>