feat: host heartbeat status and dashboard metrics (2.2)

Track agent.heartbeat and report.daily.ssh per host; show online/stale
on Hosts UI; dashboard counters and SAC_HEARTBEAT_STALE_MINUTES config.

frontend/src/style.css

@@ -160,6 +160,10 @@ pre {
   margin: 0.25rem 0 0.5rem;
 }
 
+.dash-card.dash-warn {
+  border-color: #ff6b6b;
+}
+
 .severity-list {
   list-style: none;
   padding: 0;
@@ -168,6 +172,16 @@ pre {
   gap: 1rem;
 }
 
+.agent-online {
+  color: #6bcb77;
+}
+.agent-stale {
+  color: #ff6b6b;
+}
+.agent-unknown {
+  color: #9aa4b2;
+}
+
 .live-badge {
   color: #6bcb77;
   font-size: 0.85rem;