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.

frontend/src/utils/datetime.js

@@ -0,0 +1,80 @@
+// Site-timezone datetime helpers.
+//
+// Notification start/end times are stored as UTC and must be shown + entered in
+// the site's configured IANA timezone (settings key site_timezone), not the
+// viewer's browser zone, so every site and remote admin agrees on the wall
+// clock. These convert between a UTC instant and a 'YYYY-MM-DDTHH:mm' string in
+// a given zone (the value a datetime-local input holds).
+
+const DEFAULT_TZ = 'America/New_York'
+
+function pad(n) {
+  return String(n).padStart(2, '0')
+}
+
+// Wall-clock parts of a UTC instant AS SEEN in tz. Uses Intl so it is correct
+// across DST without shipping a tz table.
+function zonedParts(date, tz) {
+  const dtf = new Intl.DateTimeFormat('en-US', {
+    timeZone: tz,
+    year: 'numeric', month: '2-digit', day: '2-digit',
+    hour: '2-digit', minute: '2-digit', second: '2-digit',
+    hourCycle: 'h23',
+  })
+  const map = {}
+  for (const p of dtf.formatToParts(date)) {
+    if (p.type !== 'literal') map[p.type] = p.value
+  }
+  return {
+    year: Number(map.year), month: Number(map.month), day: Number(map.day),
+    hour: Number(map.hour), minute: Number(map.minute), second: Number(map.second),
+  }
+}
+
+// tz offset (ms) at the given instant: (wall-clock-as-UTC) - (real UTC).
+function tzOffsetMs(date, tz) {
+  const p = zonedParts(date, tz)
+  const asUtc = Date.UTC(p.year, p.month - 1, p.day, p.hour, p.minute, p.second)
+  return asUtc - date.getTime()
+}
+
+// UTC instant (or ISO string) -> 'YYYY-MM-DDTHH:mm' wall clock in tz, for a
+// datetime-local input. Empty string on no/invalid input.
+export function zonedInputFromUtc(value, tz = DEFAULT_TZ) {
+  if (!value) return ''
+  const date = value instanceof Date ? value : new Date(value)
+  if (isNaN(date.getTime())) return ''
+  const p = zonedParts(date, tz)
+  return `${p.year}-${pad(p.month)}-${pad(p.day)}T${pad(p.hour)}:${pad(p.minute)}`
+}
+
+// 'YYYY-MM-DDTHH:mm' wall clock in tz -> UTC ISO string. Interprets the entered
+// time as tz-local (DST-aware) and returns the real instant. Empty -> null.
+export function utcFromZonedInput(localStr, tz = DEFAULT_TZ) {
+  if (!localStr) return null
+  const [datePart, timePart = '00:00'] = localStr.split('T')
+  const [y, mo, d] = datePart.split('-').map(Number)
+  const [h, mi] = timePart.split(':').map(Number)
+  if ([y, mo, d, h, mi].some(Number.isNaN)) return null
+  // Guess the instant as if the wall clock were UTC, then subtract the zone
+  // offset AT that instant. One correction is exact except in the ~1h/yr DST
+  // fold, where either side is a fair reading.
+  const guess = Date.UTC(y, mo - 1, d, h, mi)
+  const offset = tzOffsetMs(new Date(guess), tz)
+  return new Date(guess - offset).toISOString()
+}
+
+// UTC instant (or ISO) -> human string in tz, e.g. "Jul 30, 2026, 2:34 PM".
+export function formatInZone(value, tz = DEFAULT_TZ, opts = {}) {
+  if (!value) return ''
+  const date = value instanceof Date ? value : new Date(value)
+  if (isNaN(date.getTime())) return ''
+  return new Intl.DateTimeFormat('en-US', {
+    timeZone: tz,
+    year: 'numeric', month: 'short', day: 'numeric',
+    hour: 'numeric', minute: '2-digit',
+    ...opts,
+  }).format(date)
+}
+
+export { DEFAULT_TZ }

frontend/src/views/settings/SiteSettings.vue

@@ -74,6 +74,7 @@ const LABELS = {
   dualpath_single_machine: 'Dualpath as Single Machine',
   employee_directory_mode: 'Employee Directory Mode',
   usb_directory_mode: 'USB Directory Mode',
+  site_timezone: 'Site Timezone',
   setup_complete: 'Setup Complete'
 }
 function prettyLabel(key) {
@@ -83,7 +84,12 @@ function prettyLabel(key) {
 // Enumerated settings render as dropdowns to prevent typos.
 const OPTIONS = {
   employee_directory_mode: ['selfhosted', 'external'],
-  usb_directory_mode: ['selfhosted', 'external']
+  usb_directory_mode: ['selfhosted', 'external'],
+  site_timezone: [
+    'America/New_York', 'America/Chicago', 'America/Denver', 'America/Phoenix',
+    'America/Los_Angeles', 'America/Anchorage', 'Pacific/Honolulu',
+    'America/Indiana/Indianapolis'
+  ]
 }
 
 function isTrue(setting) {
@@ -97,6 +103,7 @@ const GROUPS = [
   { title: 'Behavior', keys: ['dualpath_single_machine'] },
   { title: 'Naming & Patterns', keys: ['pc_access_domain', 'printer_hostname_template', 'contact_email_domain', 'employeeid_pattern'] },
   { title: 'Data Sources', keys: ['employee_directory_mode', 'usb_directory_mode'] },
+  { title: 'Localization', keys: ['site_timezone'] },
   { title: 'System', keys: ['setup_complete'] }
 ]
 
@@ -108,7 +115,8 @@ const HELP = {
   employeeid_pattern: 'Regular expression that a scanned/typed employee ID must match to be recognized. Default: ^\\d{9}$ (9 digits). An invalid regex is ignored and the default is used.',
   printer_hostname_template: 'Template for generating printer hostnames from an IP. Use {ip} where the dash-separated IP goes. Example: Printer-{ip}.printer.geaerospace.net',
   contact_email_domain: 'Email domain appended to a support contact SSO to build email (sso@domain) and Microsoft Teams chat links. Example: geaerospace.com. Leave blank to hide the contact action buttons.',
-  dualpath_single_machine: 'Treat a Dualpath pair (a dual-bay machine with one controller) as a single machine in lists, counts, and the floor map. Both bay records are always kept; detail pages stay per-bay with a sibling banner. Enter true or false. Default: true.'
+  dualpath_single_machine: 'Treat a Dualpath pair (a dual-bay machine with one controller) as a single machine in lists, counts, and the floor map. Both bay records are always kept; detail pages stay per-bay with a sibling banner. Enter true or false. Default: true.',
+  site_timezone: 'IANA timezone for this site. Notification start/end times are shown and entered in this zone, and daily-reset notification expiry is computed here. Default: America/New_York.'
 }
 function fieldHelp(key) {
   return HELP[key] || ''