TODO

   1 Styles must be improved (full width for smartphones, selectable text for PGN...)
   2 Tooltip text should fade (even when mouse stay on it, especially for small screens)
   3 Checkered stage 2: switch button at reserve position (or on top).
   4 Mode expert: game.js, button on top (with online indicator)
   5 Turn indicator on top too (black or white)