Display row and column coordinates

[glasser]

Is your feature request related to a problem? Please describe.
When discussing a puzzle I often talk about squares by row and column like R5C8. But that requires a lot of manual counting that gets annoying. I'd love penpa to help me with it.

Describe the solution you'd like
An on-off toggle at the top of the screen that adds small "R5C8" annotations to every box.

Describe alternatives you've considered

• Something with a mouseover or special key — wouldn't be as good on mobile
• Just adding numbers to the rows and columns outside the box — better than nothing but still requires tracing lines and such.

[swaroopg92]

I have noted in the to-do list, but one of the main challenges to consider is to determine which is R1C1 because there are various possibilities of how authors set the puzzle. sometimes they add extra white space rows or columns and so on. The same goes with the last cell of the grid as well, how to determine.