Feature: Implement UTC-offset-first sorting for timezone selection dropdown

[Ubayed-Bin-Sufian]

Context

During the review of PR #90 , it was noted that while our .ics calendar export/import adheres perfectly to standards (using IANA IDs like Asia/Dhaka), the frontend dropdown presentation could be optimized for better UX.

Currently, it follows the standard Continent/City alphabetical structure. We want to transition this to a Google Calendar-style format where timezones are sorted primarily by their offset.

Reference

@Ubayed-Bin-Sufian I checked this. The calendar export/import itself follows standards (.ics + IANA timezone IDs like Asia/Dhaka), but the timezone dropdown ordering is not standardized.

The current continent/city order follows the WordPress/IANA-style timezone identifier structure. Google Calendar’s UTC-offset-first order is a UX choice, not a required standard.

So my opinion is we can merge this PR as-is and create a follow-up issue if we want to switch the timezone picker to Google Calendar-style offset ordering later.

Originally posted by @Nimeshan in #90 (comment)

Todo

• Research/implement a sorting utility to group and order timezones by UTC offset.
• Update the timezone picker UI component to reflect the new layout.