backups: show timestamps in the site timezone

Backup timestamps read wrong because of two faults stacked, which is why it
looked like a single offset.

The API serialised naive ISO ("2026-08-07T12:00:00"), with nothing saying the
value was UTC. JavaScript's new Date() parses that as BROWSER-LOCAL, so every
timestamp shifted by the viewer's offset before any timezone formatting ran.
Every datetime this plugin stores is naive UTC, so the wire format now carries
a trailing Z.

The history view then formatted with toLocaleString(), i.e. the viewer's zone,
ignoring the site_timezone setting entirely. It now loads that setting and
formats through the shared formatInZone helper, matching NotificationsList.

The panel list label is built server-side with strftime, so a client cannot
correct it afterwards. It now converts to the site zone using the same Setting
lookup the notifications plugin uses - without that it showed UTC, four hours
out at West Jefferson.

Tests cover the wire format and that 16:30Z renders as 12:30 in
America/New_York.

plugins/backups/frontend/views/BackupHistory.vue

@@ -95,7 +95,8 @@
 <script setup>
 import { ref, onMounted } from 'vue'
 import { useRoute, useRouter } from 'vue-router'
-import api from '@/api'
+import api, { settingsApi } from '@/api'
+import { formatInZone, DEFAULT_TZ } from '@/utils/datetime'
 
 const route = useRoute()
 const router = useRouter()
@@ -111,11 +112,13 @@ const diffs = ref({})
 const assetname = ref('')
 const loading = ref(true)
 const error = ref('')
+// Timestamps are stored UTC and must show in the SITE's zone, not the viewer's,
+// so a remote admin and someone at the bay read the same wall clock.
+const siteTimezone = ref(DEFAULT_TZ)
 
 function formatWhen(value) {
   if (!value) return 'unknown'
-  const d = new Date(value)
-  return isNaN(d) ? value : d.toLocaleString()
+  return formatInZone(value, siteTimezone.value, { second: '2-digit' }) || value
 }
 
 function display(value) {
@@ -124,6 +127,16 @@ function display(value) {
   return String(value)
 }
 
+async function loadTimezone() {
+  try {
+    const response = await settingsApi.get('site_timezone')
+    const value = response?.data?.data?.value
+    if (value) siteTimezone.value = value
+  } catch (e) {
+    // Keep the default zone; a missing setting must not blank the page.
+  }
+}
+
 async function load() {
   loading.value = true
   error.value = ''
@@ -183,7 +196,10 @@ async function download(rev, fmt) {
   URL.revokeObjectURL(link.href)
 }
 
-onMounted(load)
+onMounted(async () => {
+  await loadTimezone()
+  await load()
+})
 </script>
 
 <style scoped>