Thanks, glad you liked it! Undo/redo is definitely there already. The arrow buttons in the top left near the zoom buttons will do that, as well as Ctrl Z for undo and Ctrl Shift Z/Ctrl Y for redo. I totally forgot that Macs use Cmd for most shortcuts, I should be able to add that in as well, thanks for letting me know!

That being said, the undo and redo currently only handles the drawing tools, text and stamps can be deleted their own ways, but that's definitely on the todo list.