Bug: Fix page blinking after login (Chainlit#2640)

When using the `header_auth_callback ` and another auth callback for
login, the page starts to "blink" as soon as a user logged in. This is
because `header_auth_callback ` always gets called when routing to the
root.

Fix: Now when a user is already authenticated, the `header_auth_callback
`won't be called
Issue: Chainlit#2599

Author: ADWasHere

frontend/src/pages/Login.tsx

@@ -73,7 +73,7 @@ export default function Login() {
     if (!config.requireLogin) {
       navigate('/');
     }
-    if (config.headerAuth) {
+    if (config.headerAuth && !user) {
       handleHeaderAuth();
     }
     if (user) {