fix: 존재하지 않는 URL 접근 시 메인으로 리다이렉트

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

src/App.tsx

@@ -1,4 +1,4 @@
-import { BrowserRouter, Route, Routes } from 'react-router';
+import { BrowserRouter, Navigate, Route, Routes } from 'react-router';
 import { AuthProvider } from './auth/AuthProvider';
 import { ProtectedRoute } from './auth/ProtectedRoute';
 import { AdminRoute } from './auth/AdminRoute';
@@ -42,6 +42,9 @@ function App() {
                 </Route>
               </Route>
             </Route>
+
+            {/* Catch-all: 존재하지 않는 경로 → 메인으로 */}
+            <Route path="*" element={<Navigate to="/" replace />} />
           </Routes>
         </BrowserRouter>
       </AuthProvider>