[feature] Support `TZ` env var of the container/runtime to be the default timezone instead of the auto detected browser timezone

[dgtlmoon]

related to #3444

[dgtlmoon]

I renamed the field to be more obvious

Default timezone for scheduler

because the scheduler timezone might not be the same timezone as the system TZ

[dgtlmoon]

You cant use the scheduler until you verify/save the system timezone, it asks you in the UI todo that