feat: implement main navigation component and integrate protected routing in App structure

frontend/src/App.tsx

@@ -6,6 +6,7 @@ import Login from './pages/Login';
 import Dashboard from './pages/Dashboard';
 import Trending from './pages/Trending';
 import AddPodcast from './pages/AddPodcast';
+import Library from './pages/Library';
 import Navigation from './components/Navigation';
 
 const queryClient = new QueryClient({
@@ -63,6 +64,14 @@ export default function App() {
               </ProtectedRoute>
             }
           />
+          <Route
+            path="/library"
+            element={
+              <ProtectedRoute>
+                <Library />
+              </ProtectedRoute>
+            }
+          />
           <Route path="/" element={<Navigate to="/dashboard" replace />} />
           <Route path="*" element={<Navigate to="/dashboard" replace />} />
         </Routes>