feat(admin-events): event timezone picker and zoned save/load

Add a USelectMenu for displayTimezone in Event Details (defaults to
America/Toronto). On submit, convert each datetime-local string
(startDate, endDate, registrationDeadline, earlyBirdDeadline) from the
event's TZ to a UTC ISO string so the wall-clock time the admin entered
is preserved regardless of their browser TZ. On edit, render stored
UTC back through the event's TZ so the round-trip is stable.

Reuses TIMEZONE_OPTIONS from ~/config/timezones and the picker pattern
from member/profile.vue. Auto-imported helpers from app/utils/timezones
do the math via Intl.

app/pages/admin/events/create.vue

@@ -110,6 +110,23 @@
               </p>
             </div>
 
+            <div class="field">
+              <label> Event Timezone <span class="required">*</span> </label>
+              <USelectMenu
+                v-model="eventForm.displayTimezone"
+                :items="timezoneItems"
+                value-key="value"
+                searchable
+                searchable-placeholder="Search timezones..."
+                placeholder="Select a timezone"
+                class="w-full"
+              />
+              <p class="help-text">
+                Dates below are interpreted in this timezone. Attendees see the
+                event time in this zone.
+              </p>
+            </div>
+
             <div class="field">
               <label> Location <span class="required">*</span> </label>
               <UInput
@@ -601,6 +618,8 @@
 </template>
 
 <script setup>
+import { TIMEZONE_OPTIONS } from "~/config/timezones";
+
 definePageMeta({
   layout: "admin",
   middleware: "admin",
@@ -630,6 +649,7 @@ const eventForm = reactive({
   startDate: "",
   endDate: "",
   eventType: "community",
+  displayTimezone: "America/Toronto",
   location: "",
   isOnline: true,
   isVisible: true,
@@ -672,6 +692,38 @@ const formatForDatetimeLocal = (value) => {
   return `${d.getFullYear()}-${pad(d.getMonth() + 1)}-${pad(d.getDate())}T${pad(d.getHours())}:${pad(d.getMinutes())}`;
 };
 
+// Render the form's datetime fields in the event's display timezone.
+const formatForEventTZ = (value) => {
+  if (!value) return "";
+  return utcToZonedLocal(value, eventForm.displayTimezone) || formatForDatetimeLocal(value);
+};
+
+const utcOffsetLabel = (tz) => {
+  try {
+    const parts = new Intl.DateTimeFormat("en-US", {
+      timeZone: tz,
+      timeZoneName: "longOffset",
+    }).formatToParts(new Date());
+    const name = parts.find((p) => p.type === "timeZoneName")?.value || "";
+    if (name === "GMT") return "UTC+00:00";
+    return name.replace("GMT", "UTC");
+  } catch {
+    return "";
+  }
+};
+
+const timezoneItems = computed(() => {
+  const list = TIMEZONE_OPTIONS.map((t) => {
+    const off = utcOffsetLabel(t.value);
+    return { ...t, label: off ? `${t.label} (${off})` : t.label };
+  });
+  const saved = eventForm.displayTimezone;
+  if (saved && !TIMEZONE_OPTIONS.some((t) => t.value === saved)) {
+    list.unshift({ label: saved, value: saved });
+  }
+  return list;
+});
+
 // Agenda management functions
 const addAgendaItem = () => {
   eventForm.agenda.push("");
@@ -725,14 +777,17 @@ function populateEditForm(payload) {
   const event = payload?.data;
   if (!event) return;
   editingEvent.value = event;
+  // Pin the form's timezone first so subsequent date conversions use it.
+  eventForm.displayTimezone = event.displayTimezone || "America/Toronto";
   Object.assign(eventForm, {
     title: event.title,
     description: event.description,
     content: event.content || "",
     featureImage: event.featureImage || null,
-    startDate: formatForDatetimeLocal(event.startDate),
+    startDate: utcToZonedLocal(event.startDate, eventForm.displayTimezone),
-    endDate: formatForDatetimeLocal(event.endDate),
+    endDate: utcToZonedLocal(event.endDate, eventForm.displayTimezone),
     eventType: event.eventType,
+    displayTimezone: eventForm.displayTimezone,
     location: event.location || "",
     isOnline: event.isOnline,
     isVisible: event.isVisible !== undefined ? event.isVisible : true,
@@ -743,7 +798,7 @@ function populateEditForm(payload) {
     tags: event.tags || [],
     maxAttendees: event.maxAttendees || "",
     registrationRequired: event.registrationRequired,
-    registrationDeadline: formatForDatetimeLocal(event.registrationDeadline),
+    registrationDeadline: utcToZonedLocal(event.registrationDeadline, eventForm.displayTimezone),
     agenda: event.agenda || [],
     tickets: event.tickets || {
       enabled: false,
@@ -765,8 +820,9 @@ function populateEditForm(payload) {
     },
   });
   if (event.tickets?.public?.earlyBirdDeadline) {
-    eventForm.tickets.public.earlyBirdDeadline = formatForDatetimeLocal(
+    eventForm.tickets.public.earlyBirdDeadline = utcToZonedLocal(
       event.tickets.public.earlyBirdDeadline,
+      eventForm.displayTimezone,
     );
   }
 }
@@ -911,15 +967,37 @@ const saveEvent = async (redirect = true) => {
       // Individual series creation is handled through the series management page
     }
 
+    const tz = eventForm.displayTimezone || "America/Toronto";
+    const toUTC = (v) => {
+      const d = zonedLocalToUTC(v, tz);
+      return d ? d.toISOString() : v;
+    };
+    const payload = {
+      ...eventForm,
+      startDate: toUTC(eventForm.startDate),
+      endDate: toUTC(eventForm.endDate),
+      registrationDeadline: eventForm.registrationDeadline
+        ? toUTC(eventForm.registrationDeadline)
+        : eventForm.registrationDeadline,
+      tickets: {
+        ...eventForm.tickets,
+        public: {
+          ...eventForm.tickets.public,
+          earlyBirdDeadline: eventForm.tickets.public.earlyBirdDeadline
+            ? toUTC(eventForm.tickets.public.earlyBirdDeadline)
+            : eventForm.tickets.public.earlyBirdDeadline,
+        },
+      },
+    };
     if (editingEvent.value) {
       await $fetch(`/api/admin/events/${editingEvent.value._id}`, {
         method: "PUT",
-        body: eventForm,
+        body: payload,
       });
     } else {
       await $fetch("/api/admin/events", {
         method: "POST",
-        body: eventForm,
+        body: payload,
       });
     }
 
@@ -959,6 +1037,7 @@ const saveAndCreateAnother = async () => {
       startDate: "",
       endDate: "",
       eventType: "community",
+      displayTimezone: "America/Toronto",
       location: "",
       isOnline: true,
       isVisible: true,