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/models/notification.py

@@ -1,9 +1,48 @@
 """Notifications plugin models - adapted to existing database schema."""
 
 from datetime import datetime, timezone
+from zoneinfo import ZoneInfo
 from shopdb.api import db
 
 
+_DEFAULT_TZ = 'America/New_York'
+
+
+def _site_zone():
+    """Site-configured IANA zone (settings key site_timezone) for calendar day
+    placement. Imported lazily to avoid a circular import at model load."""
+    from shopdb.api import Setting
+    row = Setting.query.filter_by(key='site_timezone').first()
+    name = row.value if row and row.value else _DEFAULT_TZ
+    try:
+        return ZoneInfo(name)
+    except Exception:
+        return ZoneInfo(_DEFAULT_TZ)
+
+
+def _site_date(dt):
+    """The calendar day (YYYY-MM-DD) a stored UTC datetime falls on in the site
+    zone. allDay events must key off the site-local day, not the UTC day, or a
+    late-evening notification lands on the wrong date for western sites."""
+    if not dt:
+        return None
+    return dt.replace(tzinfo=timezone.utc).astimezone(_site_zone()).date().isoformat()
+
+
+def _utc_iso(dt):
+    """Serialize a stored datetime as an explicit-UTC ISO string.
+
+    starttime/endtime are stored NAIVE but always hold UTC wall-clock (the
+    create/update parse normalizes to UTC). Emitting a bare naive isoformat let
+    the browser read it as LOCAL time, shifting displays by the tz offset (a
+    14:34 EDT notification showed 18:34). Tag it UTC so new Date() parses the
+    real instant and renders in the viewer's zone.
+    """
+    if not dt:
+        return None
+    return dt.replace(tzinfo=timezone.utc).isoformat()
+
+
 class NotificationType(db.Model):
     """
     Notification type classification.
@@ -120,10 +159,10 @@ class Notification(db.Model):
             'notification': self.notification,
             'title': self.title,
             'message': self.notification,
-            'starttime': self.starttime.isoformat() if self.starttime else None,
-            'endtime': self.endtime.isoformat() if self.endtime else None,
-            'startdate': self.starttime.isoformat() if self.starttime else None,
-            'enddate': self.endtime.isoformat() if self.endtime else None,
+            'starttime': _utc_iso(self.starttime),
+            'endtime': _utc_iso(self.endtime),
+            'startdate': _utc_iso(self.starttime),
+            'enddate': _utc_iso(self.endtime),
             'ticketnumber': self.ticketnumber,
             'link': self.link,
             'linkurl': self.link,
@@ -175,8 +214,8 @@ class Notification(db.Model):
         return {
             'id': self.notificationid,
             'title': title,
-            'start': self.starttime.isoformat() if self.starttime else None,
-            'end': self.endtime.isoformat() if self.endtime else None,
+            'start': _site_date(self.starttime),
+            'end': _site_date(self.endtime),
             'allDay': True,
             'backgroundColor': color,
             'borderColor': color,