Add timezones for easier streak reset.

[kaiiuens]

Streak reset offset really confusing and asked DeepSeek but still confusing, so it would be a good idea to add timezones to the offset menu.

[fehmer]

The streak offset is the offset based on utc. If we add timezones and want to handle it correctly we would also need to support DST.

I think the current solution is already good enough as it shows the reset time in your local timezone (or in the timezone you configured your browser in).