Compute initial grid size to be no larger than fits on the screen

Compute initial grid size to be no larger than fits on the screen

Nobody wants a UI that starts off with a horizontal scrollbar!

But the 10x10 initial grid wasn't fitting on my mobile device. So
dynamically compute a smaller grid size that will fit.

Also, rather than hard-coding the cell size in the computation of the
grid size, use a --cell-size custom property.