User already exists message is red. Modals to the viewport center. Fixed Quit option: it deletes the session.

App.tsx

@@ -102,6 +102,7 @@ function App() {
 
   const handleStartSession = async (plan?: WorkoutPlan, startWeight?: number) => {
     if (!currentUser) return;
+    if (activeSession) return;
 
     // Get latest weight from profile or default
     const profile = getCurrentUserProfile(currentUser.id);