Improve board help interface + ctrl for yellow highlight

[Thibault-Monnier]

This PR adds ctrl as an alternative to shift while right-clicking to make a yellow highlight.

Since this increased board overlay shortcut key label width, I took advantage of the opportunity to adjust the styling of the help menu and extend its use: it is now available in sandbox mode with customized shortcuts (undo/redo only shown in sandbox, focus tile only shown in game), and is accessible through a button at the bottom-left of the screen.

As for the styling changes:

• Improved mobile view
  • Remove extra padding and make the overlay fullscreen
  • Add a scrollbar in case of overflow
• Tweak sizes
  • Keyboard and Markup sections are now styled the same, and have the same width (because the undo/redo shorcuts are large)
  • Increased gap between label and description columns
  • Made the label column size auto with a minimum of 8em and a maximum of 40%, instead of the previous fixed 8em.
• Changed the Continue Game to be white on darker background, copying the style from the Reset View button in-game.

Desktop (from sandbox):

Mobile (from game):

[Thibault-Monnier]

@WolverinDEV please consider this. And feel free to criticize if you don't like the modified styling 😀