🐛 Bad management of countdown widget on different timezones

[Tirodem]

In France :

In Kenya at the nearly same time :

We need to fix this (I think we need to pre-set a GMT/PST timezone for countdowns, challenges and leaderboards with the user setting).
While we're on it, we have to check if challenges and leaderboards end time are properly managed on multiple timezones for level progression.