notifications: correct timezone handling + configurable site timezone

Notification start/end times displayed and stored wrong by the tz offset
(a 2:34 PM entry showed 6:34 PM). Two stacked bugs: to_dict emitted stored
UTC as naive ISO (no offset) so the browser read it as local, and the form
filled the datetime-local input from toISOString() (UTC).

Fix and generalize to a configurable site timezone (multi-site):
- New setting site_timezone (default America/New_York), public, editable in
  Settings > Site > Localization (common-zone dropdown).
- Backend tags datetimes UTC (_utc_iso); parse normalizes to naive UTC
  (_parse_utc); daily-reset expiry uses the site zone (_next_site_time);
  calendar allDay events key off the site-local day (_site_date).
- Shared frontend util datetime.js (Intl-based, DST-safe) converts between a
  UTC instant and a site-zone wall clock. Notification form, list, and
  calendar all render/enter in the site zone.

plugins/notifications/frontend/views/NotificationForm.vue

@@ -169,7 +169,7 @@
                 Now
               </button>
             </div>
-            <small class="form-hint">When notification becomes visible</small>
+            <small class="form-hint">When notification becomes visible ({{ siteTimezone }})</small>
           </div>
 
           <div class="form-group">
@@ -243,8 +243,9 @@
 <script setup>
 import { ref, onMounted, computed, watch } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
-import { notificationsApi, applicationsApi, businessUnitsApi, employeesApi } from '@/api'
+import { notificationsApi, applicationsApi, businessUnitsApi, employeesApi, settingsApi } from '@/api'
 import { apiError } from '@/utils/apiError'
+import { zonedInputFromUtc, utcFromZonedInput, DEFAULT_TZ } from '@/utils/datetime'
 
 const route = useRoute()
 const router = useRouter()
@@ -278,6 +279,10 @@ const form = ref({
   employeesso: ''
 })
 
+// Site timezone (settings key site_timezone) so form times show/enter in the
+// site's wall clock, not the viewer's browser zone. Loaded in onMounted.
+const siteTimezone = ref(DEFAULT_TZ)
+
 function selectedTypeName() {
   const selectedType = types.value.find(t => t.notificationtypeid === parseInt(form.value.notificationtypeid))
   return selectedType?.typename?.toLowerCase() || ''
@@ -303,13 +308,18 @@ const messagePlaceholder = computed(() =>
 
 onMounted(async () => {
   try {
-    // Load dropdown data in parallel
-    const [typesRes, buRes, appsRes] = await Promise.all([
+    // Load dropdown data + the site timezone in parallel. Resolve the tz FIRST
+    // (before any formatDateForInput) so times render in the site's zone.
+    const [typesRes, buRes, appsRes, tzRes] = await Promise.all([
       notificationsApi.types.list(),
       businessUnitsApi.list().catch(() => ({ data: { data: [] } })),
-      applicationsApi.list({ perpage: 500 }).catch(() => ({ data: { data: [] } }))
+      applicationsApi.list({ perpage: 500 }).catch(() => ({ data: { data: [] } })),
+      settingsApi.get('site_timezone').catch(() => null)
     ])
 
+    const tzValue = tzRes?.data?.data?.value
+    if (tzValue) siteTimezone.value = tzValue
+
     types.value = typesRes.data.data || []
     businessUnits.value = buRes.data?.data || []
     applications.value = appsRes.data?.data || []
@@ -364,14 +374,13 @@ onMounted(async () => {
   }
 })
 
+// UTC value from the API -> a datetime-local string in the SITE timezone.
 function formatDateForInput(dateStr) {
-  if (!dateStr) return ''
-  const date = new Date(dateStr)
-  return date.toISOString().slice(0, 16)
+  return zonedInputFromUtc(dateStr, siteTimezone.value)
 }
 
 function setNow(field) {
-  form.value[field] = formatDateForInput(new Date().toISOString())
+  form.value[field] = zonedInputFromUtc(new Date(), siteTimezone.value)
 }
 
 function onTypeChange() {
@@ -379,7 +388,7 @@ function onTypeChange() {
   // (recognition clears at 8 AM Eastern; recertification runs two weeks). Set
   // start to now and leave end blank so the backend applies the per-type rule.
   if (isEmployeeType.value) {
-    form.value.starttime = formatDateForInput(new Date().toISOString())
+    form.value.starttime = zonedInputFromUtc(new Date(), siteTimezone.value)
     form.value.endtime = ''
   }
 }
@@ -481,8 +490,8 @@ async function saveNotification() {
       appid: parseInt(form.value.appid) || null,
       ticketnumber: form.value.ticketnumber || null,
       link: form.value.link || null,
-      starttime: form.value.starttime ? new Date(form.value.starttime).toISOString() : null,
-      endtime: form.value.endtime ? new Date(form.value.endtime).toISOString() : null,
+      starttime: utcFromZonedInput(form.value.starttime, siteTimezone.value),
+      endtime: utcFromZonedInput(form.value.endtime, siteTimezone.value),
       isactive: form.value.isactive,
       isshopfloor: form.value.isshopfloor,
       employeesso: form.value.employeesso || null