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/CalendarView.vue

@@ -231,13 +231,17 @@ function openEventFromTooltip(evt) {
 
 function formatDate(dateStr) {
   if (!dateStr) return ''
-  return new Date(dateStr).toLocaleDateString('en-US', {
+  // allDay events carry a site-local date-only value (YYYY-MM-DD). Build the
+  // Date from local parts so it is not shifted a day by UTC-midnight parsing.
+  const parts = /^(\d{4})-(\d{2})-(\d{2})$/.exec(dateStr)
+  const date = parts
+    ? new Date(Number(parts[1]), Number(parts[2]) - 1, Number(parts[3]))
+    : new Date(dateStr)
+  return date.toLocaleDateString('en-US', {
     weekday: 'short',
     year: 'numeric',
     month: 'short',
-    day: 'numeric',
-    hour: '2-digit',
-    minute: '2-digit'
+    day: 'numeric'
   })
 }
 </script>