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/artist/ArtistHeader.vue

@@ -35,6 +35,7 @@
 
 <script setup>
 import { computed } from 'vue'
+import { formatLocalDate } from '../../utils/date.js'
 
 const props = defineProps({
   name:        { type: String, required: true },
@@ -52,7 +53,7 @@ const stats = computed(() => {
     parts.push(`${props.imageCount} image${props.imageCount === 1 ? '' : 's'}`)
   }
   if (props.lastAdded) {
-    parts.push(`last added ${props.lastAdded.slice(0, 10)}`)
+    parts.push(`last added ${formatLocalDate(props.lastAdded)}`)
   }
   return parts.join(' · ')
 })