feat(icsExport): add IANA timezone support for calendar events

[rupesh-kumar-sah]

Summary of Changes

Resolves #703

This PR adds IANA timezone support to icsExport:

1. Timezone Parameter & Options Support:
   • generateICS accepts an optional timezone string (or ICSOptions object with timezone and dtstamp).
2. VTIMEZONE Component & DTSTART;TZID:
   • When a timezone (e.g. "America/New_York") is supplied, a standard RFC 5545 VTIMEZONE component is generated with computed offset and timezone name.
   • Event DTSTART is formatted as DTSTART;TZID=<timezone>:<localDateTime>.
3. UTC Fallback:
   • When timezone is omitted, the exporter cleanly falls back to UTC / DTSTART;VALUE=DATE (preserving existing behavior).
4. Verification:
   • Unit tests added for timezone support via parameter and options object, as well as UTC fallback.
   • 10/10 tests pass, tsc --noEmit clean.

[vercel]

@rupesh-kumar-sah is attempting to deploy a commit to the kingsman-99's projects Team on Vercel.

A member of the Team first needs to authorize it.