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/downloads/DownloadDetailModal.vue

@@ -7,7 +7,7 @@
       </v-card-title>
       <v-card-text>
         <p class="text-caption" style="opacity: 0.75">
-          {{ event.started_at }} → {{ event.finished_at || '(running)' }}
+          {{ formatDateTime(event.started_at) }} → {{ event.finished_at ? formatDateTime(event.finished_at) : '(running)' }}
           ({{ fmtDuration(event.summary?.duration_seconds) }})
         </p>
 
@@ -105,6 +105,7 @@ import { toast } from '../../utils/toast.js'
 import { computed } from 'vue'
 
 import { copyText } from '../../utils/clipboard.js'
+import { formatDateTime } from '../../utils/date.js'
 
 const props = defineProps({ event: { type: Object, default: null } })
 const emit = defineEmits(['close'])