refactor: update user timezone retrieval in get_user function

[iamimmanuelraj]

Problem

When creating a helpdesk ticket the users sees it as a action in the past but its happend recently. This is because the frontend is using timezone from the user. In this case timezone of the user is same as the system timezone as well but still this occoured. Ideally this should should show the same x minutes ago in all timezone but it is not working that way. Since the document creation and other time is based on the system time instead of the user time its better to use system's time itself in all place to fix that issue

Screen.Recording.2025-08-05.at.11.44.26.AM.mov

Changelog

This pull request updates how the user's time zone is handled in the get_user function in helpdesk/api/auth.py. Instead of retrieving the user's individual time zone from the database, the system-wide time zone is now used.

User time zone handling:

• Removed retrieval of the user's personal time_zone field from the User doctype and replaced it with the system-wide time zone using get_system_timezone() in the returned user dictionary. [1] [2]
• Added an import for get_system_timezone from frappe.utils to support the new time zone logic.

[iamimmanuelraj]

@RitvikSardana Can you check this and merge it ASAP? We are having issues on our production. We are running application that is operated from multiple regions