Add login route handling in AppRouter and introduce SessionRouter for session management

frontend/src/App.tsx

@@ -332,6 +332,13 @@ function App() {
 }
 
 function AppRouter() {
+  if (window.location.pathname === '/login') {
+    return <RouterProvider router={router} context={{ session: null }} />
+  }
+  return <SessionRouter />
+}
+
+function SessionRouter() {
   const session = useQuery({
     queryKey: ['session'],
     queryFn: api.session,