Show the last event as a relative time with a UTC tooltip (closes #567)

The Statistics block's "Last event" figure now shows how long ago the
newest event arrived, as its two largest units ("59s", "1m1s", "3d4h"),
worked out when the page is served. Its tooltip is the full time in
UTC. With no event it still shows "none", without a tooltip.

Model: opus-5-5

internal/handlers/export_test.go

@@ -96,6 +96,12 @@ func (s *Handlers) WebhookStatsForTest(webhookID string) *WebhookStats {
 	return s.loadWebhookStats(webhookID, entrypoints, targets)
 }
 
+// RelativeTimeForTest exposes relativeTime for use in the
+// handlers_test package.
+func RelativeTimeForTest(at, now time.Time) string {
+	return relativeTime(at, now)
+}
+
 // FinishedByTargetForTest exposes finishedByTarget for use in the
 // handlers_test package.
 func FinishedByTargetForTest(