Refactor: centralize timezone utilities in src/utils/timezone.ts (#491)

Deduplicate resolveTimezone, resolveTimezoneId, convertEventDateTimeToISO,
and getTimezoneOffset into a single canonical module, removing copies from
EventApi, eventUtils, TimezoneSelector, EventUpdateModal, and calendarUtils.

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/features/Events/EventModal.tsx

@@ -1,8 +1,5 @@
 import { useAppDispatch, useAppSelector } from "@/app/hooks";
-import {
-  getTimezoneOffset,
-  resolveTimezone,
-} from "@/components/Calendar/TimezoneSelector";
+import { getTimezoneOffset, resolveTimezone } from "@/utils/timezone";
 import { updateTempCalendar } from "@/components/Calendar/utils/calendarUtils";
 import { ResponsiveDialog } from "@/components/Dialog";
 import EventFormFields from "@/components/Event/EventFormFields";