TODO

   1 Finish rules translation in Spanish + improve existing ones
   2 Design: final touch (gain extra space on top, using space on the right)
   3 Crazyhouse: center reserves, grey if zero available, numbers superimposed
   4 Promotions: increase pieces sizes, better background.
   5 Code: use two spaces instead of tabs, everywhere.
   6 Increase code line length to 100 or more?
   7 (http://katafrakt.me/2017/09/16/80-characters-line-length-limit/)
   8 Chat button should be more apparent after game ends (color ?)
   9 Reinforce security for problems upload (how ?)
  10 The mode switch between human/computer/friend (+ problem) is a mess
  11 (example: finished computer game, ongoing friend game, reload, friend game is unreachable)
  12
  13 Later:
  14 Let choice of time control, allow correspondance play, several games at the same time
  15 ==> need to use indexedDB instead of localStorage. Maybe with Dexie https://dexie.org/
  16 Each user would have a unique identifier stored in the client DB.
  17 Allow to cancel games (if opponent doesn't connect again)
  18 Identity would be browser-based: different games on smartphone, home computer, work computer... (why not ?)
  19 Index might still look the same, and variant page would have another tab "Games"
  20 ==> running, and finished (which can be deleted from local memory)
  21 (A true analysis mode could be implemented also, to navigate in completed games --> use a button)
  22 Allow challenging a specific player (by his chosen name)
  23 But keep the random pairings as main playing way + always playing in ZEN mode,
  24 except when accepting an individual challenge.