[ENHANCEMENT] Expand timezone options to cover all IANA timezones

[BenGWeeks]

Description

The current profile page has a hardcoded list of ~14 major timezones in TIMEZONE_OPTIONS. This may not cover all users globally.

Suggested Implementation

Use Intl.supportedValuesOf("timeZone") to dynamically generate a complete list of IANA timezones. This provides:

• Full coverage of all valid timezones
• Automatic updates as browser support evolves
• No maintenance of hardcoded lists

Example

const TIMEZONE_OPTIONS = Intl.supportedValuesOf("timeZone");

Related

Identified during PR #108 code review:
#108 (comment)

Acceptance Criteria

• Replace hardcoded timezone list with dynamic generation
• Ensure timezone dropdown remains performant with larger list
• Consider adding search/filter functionality for easier selection