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/utils/date.js

@@ -32,3 +32,32 @@ export function formatRelative(iso, opts = {}) {
   if (future) return diff <= 0 ? 'imminent' : `in ${body}`
   return `${body} ago`
 }
+
+// Parse a backend timestamp as a UTC instant. Backend serializes tz-aware
+// UTC (…+00:00); if a value ever arrives without a tz designator we treat
+// it as UTC so it still converts to the viewer's zone correctly.
+function _parseUtc (iso) {
+  if (typeof iso !== 'string' || !iso) return null
+  const hasTz = /([zZ])$|([+-]\d{2}:?\d{2})$/.test(iso)
+  const d = new Date(hasTz ? iso : `${iso}Z`)
+  return isNaN(d.getTime()) ? null : d
+}
+
+// Local-timezone date + time, e.g. "May 28, 7:30 PM". Use for absolute
+// timestamps that were previously shown as raw UTC ISO.
+export function formatDateTime (iso) {
+  const d = _parseUtc(iso)
+  if (!d) return ''
+  return d.toLocaleString(undefined, {
+    month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit',
+  })
+}
+
+// Local-timezone date only, e.g. "May 28, 2026".
+export function formatLocalDate (iso) {
+  const d = _parseUtc(iso)
+  if (!d) return ''
+  return d.toLocaleDateString(undefined, {
+    year: 'numeric', month: 'short', day: 'numeric',
+  })
+}