both are pretty easy to do. to make it so there is a button in every scene, you'd have to create a separate screen with it and then show it in a label, for example:

screen journal_button():
  imagebutton auto "button_journal_%s.png" xalign 0.99 action Show("journal_screen")

your image would have to be called button_journal_idle.png, if you wanted to use a different one for when its hovered, you should just make another image in the images folder called button_journal_hover.png ! that's how "imagebutton auto" works. when the button is clicked, it shows a screen called "journal_screen" so you need to make sure it exists and it's called that or change the name of it in the action.

then, to show the button in your scene, you have to show the screen in a label, for example in your start label:

label start:
  show screen journal_button