fix: use preferred_username instead of username

app.py

@@ -255,7 +255,7 @@ def hello(profile: gr.OAuthProfile | None) -> str:
         NAME = "guest"
         return "I don't know you. Please login:"
     NAME = profile.name
-    USERNAME = profile.username
+    USERNAME = profile.preferred_username
     return f"Hello {profile.name}"
     
 with gr.Blocks(css=CSS) as demo: