icsExport: Add IANA timezone support for calendar events

[Kingsman-99]

Description

icsExport generates .ics calendar files for payment due dates but writes all times in UTC without a TZID property, causing events to appear at the wrong local time in many calendar apps. Contributors should add a timezone parameter (IANA tz string) and embed the correct DTSTART;TZID=... and VTIMEZONE component in the generated ICS data.

Acceptance Criteria

• A timezone parameter (IANA string, e.g. "America/New_York") is accepted by the export function
• Generated events include DTSTART;TZID=<tz> and a VTIMEZONE block for the given timezone
• When timezone is omitted the export falls back to UTC (current behaviour)
• Existing tests continue to pass

Context

• Target file: src/lib/icsExport.ts
• Date helpers: src/lib/dateUtils.ts

[damiedee96]

Hello Maintainer, I'd love to work on this issue.