feat(provenance): locale-independent formatPostDate util

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

frontend/src/utils/date.js

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+// 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()}`
+}

frontend/test/date.spec.js

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+import { describe, it, expect } from 'vitest'
+import { formatPostDate } from '../src/utils/date.js'
+
+describe('formatPostDate', () => {
+  it('formats an ISO datetime as "Mon D, YYYY" in UTC', () => {
+    expect(formatPostDate('2023-08-01T00:00:00+00:00')).toBe('Aug 1, 2023')
+    expect(formatPostDate('2023-12-25T23:59:59Z')).toBe('Dec 25, 2023')
+  })
+
+  it('returns null for null/empty/invalid', () => {
+    expect(formatPostDate(null)).toBe(null)
+    expect(formatPostDate('')).toBe(null)
+    expect(formatPostDate('not-a-date')).toBe(null)
+  })
+})