Events stored UTC have always been interpreted in viewer-local TZ at
the admin form and render layer. Adding an event-owned IANA timezone
unblocks accurate scheduling and display regardless of the admin's or
viewer's browser TZ.
- Event.displayTimezone (default "America/Toronto") on the model.
- displayTimezone added to admin create/update Zod schemas.
- app/utils/timezones.js: zonedLocalToUTC, utcToZonedLocal,
shortTimezoneName — Intl-based helpers, no new dependencies.
