feat(events): render public detail page in event timezone

Event detail page formatted dates in viewer-local time, so a Lisbon
viewer of a Toronto ET event saw the time in WEST. Format with
event.displayTimezone instead so attendees see the event's intended
wall-clock + zone suffix ("6:00 AM EDT") regardless of where they sit.

useEventDateUtils.formatDate / formatTime / formatDateRange / isToday
now accept a { timeZone } option and pass it to Intl.DateTimeFormat.
Existing call sites that don't pass timeZone fall through to viewer-
local, matching prior behaviour.

app/pages/events/[slug].vue

@@ -197,21 +197,29 @@ onMounted(async () => {
   }
 });
 
+const eventTimeZone = computed(
+  () => event.value?.displayTimezone || "America/Toronto",
+);
+
 const formatDate = (dateStr) => {
+  if (!dateStr) return "";
   const d = new Date(dateStr);
   return new Intl.DateTimeFormat("en-US", {
     weekday: "long",
     year: "numeric",
     month: "long",
     day: "numeric",
+    timeZone: eventTimeZone.value,
   }).format(d);
 };
 
 const formatTime = (start, end) => {
+  if (!start || !end) return "";
   const fmt = new Intl.DateTimeFormat("en-US", {
     hour: "numeric",
     minute: "2-digit",
     timeZoneName: "short",
+    timeZone: eventTimeZone.value,
   });
   return `${fmt.format(new Date(start))} – ${fmt.format(new Date(end))}`;
 };