fix(ui): render absolute timestamps in the viewer's local timezone

Download event times showed raw UTC wall-clock (iso.slice). Added
formatDateTime()/formatLocalDate() (local tz, robust to naive vs tz-aware
ISO) and applied them to the download row + detail modal datetimes and the
credential/artist date displays. formatPostDate stays UTC (date-only,
locale-stable, unit-tested).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/subscriptions/CredentialCard.vue

@@ -102,6 +102,7 @@ import { toast } from '../../utils/toast.js'
 import { computed, ref } from 'vue'
 import PlatformChip from './PlatformChip.vue'
 import { useCredentialsStore } from '../../stores/credentials.js'
+import { formatLocalDate } from '../../utils/date.js'
 
 const props = defineProps({
   platform: { type: Object, required: true },
@@ -195,8 +196,7 @@ const howToFallback = computed(() => {
 })
 
 function fmtDate(iso) {
-  if (!iso) return '—'
-  return iso.slice(0, 10)
+  return iso ? formatLocalDate(iso) : '—'
 }
 </script>