Refactored calendarfetcherutils to fix many of the timezone and DST related issues and make debugging way easier (#3806)

Refactored calendarfetcherutils to remove as many of the date
conversions as possible and use moment tz when calculating recurring
events, this will make debugging a lot easier and fixes problems from
the past with offsets and DST not being handled properly. Also added
some tests to test the behavior of the refactored methodes to make sure
the correct event dates are returned.

Refactored calendar.js aswell to make sure the unix UTC start and end
date of events are properly converted to a local timezone and displayed
correctly for the user.

This PR relates to:
https://github.com/MagicMirrorOrg/MagicMirror/issues/3797

---------

Co-authored-by: Koen Konst <c.h.konst@avisi.nl>
Co-authored-by: Kristjan ESPERANTO <35647502+KristjanESPERANTO@users.noreply.github.com>

CHANGELOG.md

@@ -34,6 +34,10 @@ planned for 2025-07-01
 - [refactor] Replace `ansis` with built-in function `util.styleText` (#3793)
 - [core] Integrate stuff from `vendor` and `fonts` folders into main `package.json`, simplifies install and maintaining dependencies (#3795, #3805)
 - [l10n] Complete translations (with the help of translation tools) (#3794)
+- [refactor] Refactored `calendarfetcherutils` in Calendar module to handle timezones better (#3806)
+  - Removed as many of the date conversions as possible
+  - Use `moment-timezone` when calculating recurring events, this will fix problems from the past with offsets and DST not being handled properly
+  - Added some tests to test the behavior of the refactored methods to make sure the correct event dates are returned
 
 ### Fixed