daily Ui 004: calculator

Not too many changes made to the existing iPhone calculator layout, but a big pain point (at least personally) is the inability to backtrack during calculations, which gets frustrating when there is a wrong input within a long string of calculations or forgetting previous inputs.

To alleviate this, I added a "delete" button to make changes to an existing input, as well as the ability to view/undo previous calculations by putting strings above the current result.