feat(alerts/ui): add formatRelativeTime helper

Formats ISO timestamps as `Nm ago` / `Nh ago` / `Nd ago`, falling back
to an absolute locale date string for values older than 30 days. Used
by the alert DataTable Age column.

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

ui/src/pages/Alerts/time-utils.test.ts

@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { formatRelativeTime } from './time-utils';
+
+const NOW = new Date('2026-04-21T12:00:00Z');
+
+describe('formatRelativeTime', () => {
+  it('returns "just now" for < 30s', () => {
+    expect(formatRelativeTime('2026-04-21T11:59:50Z', NOW)).toBe('just now');
+  });
+
+  it('returns minutes for < 60m', () => {
+    expect(formatRelativeTime('2026-04-21T11:57:00Z', NOW)).toBe('3m ago');
+  });
+
+  it('returns hours for < 24h', () => {
+    expect(formatRelativeTime('2026-04-21T10:00:00Z', NOW)).toBe('2h ago');
+  });
+
+  it('returns days for < 30d', () => {
+    expect(formatRelativeTime('2026-04-18T12:00:00Z', NOW)).toBe('3d ago');
+  });
+
+  it('returns locale date string for older than 30d', () => {
+    const out = formatRelativeTime('2025-01-01T00:00:00Z', NOW);
+    expect(out).not.toMatch(/ago$/);
+    expect(out.length).toBeGreaterThan(0);
+  });
+
+  it('handles future timestamps by clamping to "just now"', () => {
+    expect(formatRelativeTime('2026-04-21T12:00:30Z', NOW)).toBe('just now');
+  });
+});

ui/src/pages/Alerts/time-utils.ts

@@ -0,0 +1,12 @@
+export function formatRelativeTime(iso: string, now: Date = new Date()): string {
+  const then = new Date(iso).getTime();
+  const diffSec = Math.max(0, Math.floor((now.getTime() - then) / 1000));
+  if (diffSec < 30) return 'just now';
+  if (diffSec < 3600) return `${Math.floor(diffSec / 60)}m ago`;
+  if (diffSec < 86_400) return `${Math.floor(diffSec / 3600)}h ago`;
+  const diffDays = Math.floor(diffSec / 86_400);
+  if (diffDays < 30) return `${diffDays}d ago`;
+  return new Date(iso).toLocaleDateString('en-GB', {
+    year: 'numeric', month: 'short', day: '2-digit',
+  });
+}