feat(events): add displayTimezone field and zoned datetime helpers

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.

server/models/event.js

@@ -18,6 +18,8 @@ const eventSchema = new mongoose.Schema({
     enum: ["community", "workshop", "social", "showcase"],
     default: "community",
   },
+  // IANA timezone for interpreting datetime input and rendering the event time.
+  displayTimezone: { type: String, default: "America/Toronto" },
   // Online-first location handling
   location: {
     type: String,