Allow user to zoom

[grantm]

The app currently monitors resize events on the window object and scales the grid and virtual keyboard to fit within the available client area of the window.

It's possible that a user with a visual impairment might want to zooms in (say with pinch to zoom on a mobile device) on an area of the screen in order to more easily read the numbers. However the app doesn't currently allow this.

If the app was zoomed in, the distinction between dragging to pan around vs dragging to select cells might become confusing. But then again it might not. This feature could be hidden behind an "Allow zoom" option if UI weirdness resulted.