little bit of login checking

...pp/src/main/java/com/example/schedulerapp/Controller.java

...ulerApp/src/main/java/com/example/schedulerapp/Model.java

...p/src/main/java/com/example/schedulerapp/ScheduleApp.java

.../src/main/java/com/example/schedulerapp/ScheduleView.java

...ain/resources/com/example/schedulerapp/headerManager.fxml

...pp/src/main/resources/com/example/schedulerapp/login.fxml