Add forward and back arrows to header

[akf]

This is just a "would be nice," but it's the only thing that occurs to me that I'd add feature-wise. Currently, to go from one day to the next, I have to go to the menu and navigate. Adding the navigation to the header would make it much smoother and is probably (?) easy.

Other navigation memes I tried and that people might want are simply scrolling past one day to the next, or swiping sideways to go forward and back.