Time.here is missing eras

[sldab]

Problem

Time.here captures the current timezone offset with an empty eras list [].

Impact

When you're in CET (UTC+1, winter), it creates a zone fixed at +60 minutes. Displaying a summer date (which should use CEST, UTC+2) still uses +60 minutes => off by one hour.

Proposed solution

Get the user's timezone with past and future DST era information in the Kernel's JS code by scanning monthly from 1970 to 2100 and calling getTimezoneOffset() repeatedly to detect DST transitions:
https://github.com/gren-lang/core/blob/main/src/Gren/Kernel/Time.js#L25C1-L33C2