Finish rules translation in Spanish + improve existing ones
Design: final touch (gain extra space on top, using space on the right)
Crazyhouse: center reserves, grey if zero available, numbers superimposed
Promotions: increase pieces sizes, better background.
Code: use two spaces instead of tabs, everywhere.
Increase code line length to 100 or more?
(http://katafrakt.me/2017/09/16/80-characters-line-length-limit/)
Chat button should be more apparent after game ends (color ?)
Reinforce security for problems upload (how ?)
The mode switch between human/computer/friend (+ problem) is a mess
(example: finished computer game, ongoing friend game, reload, friend game is unreachable)

Later:
Let choice of time control, allow correspondance play, several games at the same time
==> need to use indexedDB instead of localStorage. Maybe with Dexie https://dexie.org/
Each user would have a unique identifier stored in the client DB.
Allow to cancel games (if opponent doesn't connect again)
Identity would be browser-based: different games on smartphone, home computer, work computer... (why not ?)
Index might still look the same, and variant page would have another tab "Games"
==> running, and finished (which can be deleted from local memory)
(A true analysis mode could be implemented also, to navigate in completed games --> use a button)
Allow challenging a specific player (by his chosen name)
But keep the random pairings as main playing way + always playing in ZEN mode,
except when accepting an individual challenge.