Add timezone selection to new UI

[bbovenzi]

Let users specify their preferred timezone.

Add a Timezone context which reads and write to local storage, and passes the selected timezone down to all <Time /> components. Also, always show UTC and the guessed TZ at the top of the search list.

Closes #42817
Related #42821

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[jscheffl]

Cool!

Tested and all look good!

The only think that I favor from the previous UI: Less clicks needed to switch between time zones :-)

[bbovenzi]

Cool!

Tested and all look good!

The only think that I favor from the previous UI: Less clicks needed to switch between time zones :-)

I noticed that too with moving light/dark mode inside user settings. So I might try to redesign that

[jscheffl]

Cool!
Tested and all look good!
The only think that I favor from the previous UI: Less clicks needed to switch between time zones :-)

I noticed that too with moving light/dark mode inside user settings. So I might try to redesign that

But no force to have this in this PR... can also be a follow-up optimization. Critical functions in sceleton are more important (I think...)