fix(auth): change username claim to preferred_username

internal/auth/routes.go

@@ -78,7 +78,15 @@ func CallbackHandler(mux *http.ServeMux) {
 			return
 		}
 
-		jwt, err := NewJWT(idtoken.Subject)
+		var UsernameClaim struct {
+			PreferredUsername string `json:"preferred_username"`
+		}
+		if err := idtoken.Claims(&UsernameClaim); err != nil {
+			http.Error(w, err.Error(), http.StatusBadRequest)
+			return
+		}
+
+		jwt, err := NewJWT(UsernameClaim.PreferredUsername)
 
 		if err != nil {
 			http.Error(w, err.Error(), http.StatusInternalServerError)