feat(monitor): add STALE state for push monitors

New intermediate state between LATE and DOWN at the midpoint of
the grace period. Gives operators earlier warning that a push
monitor has gone quiet. Includes dedicated orange theme color
across all 5 themes and proper styling in dashboard, detail
panel, and history view.

internal/monitor/monitor.go

@@ -509,6 +509,7 @@ func (e *Engine) checkPush(site models.Site) {
 	}
 
 	overdue := site.LastCheck.Add(interval)
+	staleMark := overdue.Add(grace / 2)
 	graceEnd := overdue.Add(grace)
 	now := time.Now()
 
@@ -516,6 +517,10 @@ func (e *Engine) checkPush(site models.Site) {
 		if site.Status != "DOWN" {
 			e.handleStatusChange(site, "DOWN", 0, 0, "heartbeat missed")
 		}
+	} else if now.After(staleMark) {
+		if site.Status != "STALE" {
+			e.handleStatusChange(site, "STALE", 0, 0, "heartbeat stale")
+		}
 	} else if now.After(overdue) {
 		if site.Status != "LATE" {
 			e.handleStatusChange(site, "LATE", 0, 0, "heartbeat overdue")