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.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',
+  });
+}