feat(provenance): locale-independent formatPostDate util

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

frontend/src/utils/date.js

@@ -0,0 +1,15 @@
+// Locale-independent post-date formatting. Uses a fixed month table and
+// UTC getters so the rendered string never varies by browser/CI locale
+// or timezone (toLocaleDateString would make tests flaky).
+
+const MONTHS = [
+  'Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
+  'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'
+]
+
+export function formatPostDate(iso) {
+  if (!iso) return null
+  const d = new Date(iso)
+  if (isNaN(d.getTime())) return null
+  return `${MONTHS[d.getUTCMonth()]} ${d.getUTCDate()}, ${d.getUTCFullYear()}`
+}