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/credentials/PlatformCredentialRow.vue

@@ -32,6 +32,8 @@
 </template>
 
 <script setup>
+import { formatLocalDate } from '../../utils/date.js'
+
 defineProps({
   platform: { type: Object, required: true },
   credential: { type: Object, default: null },
@@ -39,8 +41,7 @@ defineProps({
 defineEmits(['replace', 'remove'])
 
 function fmtDate(iso) {
-  if (!iso) return '—'
-  return iso.slice(0, 10)
+  return iso ? formatLocalDate(iso) : '—'
 }
 </script>