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.

shopdb/core/api/settings.py

@@ -56,6 +56,7 @@ PUBLIC_SETTING_CATEGORIES = {'branding', 'map'}
 PUBLIC_SETTING_KEYS = {
     'site_base_url', 'facility_name', 'printer_hostname_template',
     'contact_email_domain', 'servicenow_enabled', 'setup_complete',
+    'site_timezone',
 }
 
 
@@ -443,6 +444,13 @@ def build_default_settings():
             'category': 'site',
             'description': 'Set true once the first-run setup wizard has been finished'
         },
+        {
+            'key': 'site_timezone',
+            'value': 'America/New_York',
+            'valuetype': 'string',
+            'category': 'site',
+            'description': "IANA timezone for this site (e.g. America/New_York, America/Chicago). Notification start/end times and daily-reset expiry are computed in this zone."
+        },
         {
             'key': 'employee_directory_mode',
             'value': 'selfhosted',