refactor: src 루트 파일 정리

- App.jsx 404 인라인 JSX → NotFoundPage 컴포넌트 분리, <a> → <Link> 수정
- index.css btn-game transition brightness → filter 오타 수정

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

src/App.jsx

@@ -11,6 +11,7 @@ import HomePage from './pages/HomePage';
 import AdminPage from './pages/AdminPage';
 import WalletPage from './pages/WalletPage';
 import SSAFYCallbackPage from './pages/SSAFYCallbackPage';
+import NotFoundPage from './pages/NotFoundPage';
 
 function AuthRedirect() {
   const { user } = useAuth();
@@ -66,13 +67,7 @@ function AppRoutes() {
             </AdminRoute>
           }
         />
-        <Route path="*" element={
+        <Route path="*" element={<NotFoundPage />} />
-          <div style={{ padding: '2rem', textAlign: 'center', color: '#aaa' }}>
-            <h2>404</h2>
-            <p>페이지를 찾을 수 없습니다</p>
-            <a href="/" style={{ color: '#4ea8de' }}>홈으로 돌아가기</a>
-          </div>
-        } />
       </Routes>
     </ErrorBoundary>
   );

src/index.css

@@ -45,7 +45,7 @@ a:hover {
   background: url('/images/btn_normal.webp') center/100% 100% no-repeat;
   letter-spacing: 0.05em;
   overflow: hidden;
-  transition: transform 0.15s, brightness 0.15s;
+  transition: transform 0.15s, filter 0.15s;
 }
 .btn-game:hover { transform: translateY(-1px); filter: brightness(1.2); }
 .btn-game:disabled { opacity: 0.5; cursor: not-allowed; transform: none; }

src/pages/NotFoundPage.jsx

@@ -0,0 +1,11 @@
+import { Link } from 'react-router-dom';
+
+export default function NotFoundPage() {
+  return (
+    <div style={{ padding: '2rem', textAlign: 'center', color: '#aaa' }}>
+      <h2>404</h2>
+      <p>페이지를 찾을 수 없습니다</p>
+      <Link to="/" style={{ color: '#4ea8de' }}>홈으로 돌아가기</Link>
+    </div>
+  );
+}