#172: floor sidecar-down's age at the window when no Lane ever reached the sidecar

A fresh database whose browser Lanes skipped from day one has no
sidecar-reaching pass, so sidecarDownSince returned Since 0 and the notice
rendered the age since the epoch. Report the window itself: 'at least
twelve hours' is the honest floor the condition guarantees.

backend/internal/latest/faults.go

@@ -146,6 +146,12 @@ func sidecarDownSince(passes []store.LanePass, ok map[string]int64, cutoff int64
 			since = reached
 		}
 	}
+	// No Lane's retained pass log shows a sidecar reach: the honest age is
+	// the window itself — "at least twelve hours" — not the epoch, which
+	// humanAge would render as tens of thousands of days.
+	if since == 0 {
+		since = cutoff
+	}
 	return since, true
 }