feat: 앱 셸 모바일 레이아웃 — BottomNav 통합 + 사이드바 조건부 렌더링

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.jsx

@@ -1,11 +1,15 @@
 import React from 'react';
 import { Outlet } from 'react-router-dom';
 import Navbar from './components/Navbar';
+import BottomNav from './components/BottomNav';
 import PageHeader from './components/PageHeader';
 import Loading from './components/Loading';
+import { useIsMobile } from './hooks/useIsMobile';
 import './App.css';
 
 function App() {
+    const isMobile = useIsMobile();
+
     return (
         <div className="app-shell">
             <Navbar />
@@ -17,6 +21,7 @@ function App() {
                     </React.Suspense>
                 </main>
             </div>
+            {isMobile && <BottomNav />}
         </div>
     );
 }