Improve host disconnection detection and fix notification exclusions

- Add dedicated heartbeat transmission every 5 seconds independent of metric collection
- Fix host offline detection by clearing metrics for disconnected hosts
- Move exclude_email_metrics to NotificationConfig for better organization
- Add cleanup_offline_hosts method to remove stale metrics after heartbeat timeout
- Ensure offline hosts show proper status icons and visual indicators

Version 0.1.63

dashboard/src/app.rs

@@ -278,11 +278,13 @@ impl Dashboard {
 
             // Check for host connectivity changes (heartbeat timeouts) periodically
             if last_heartbeat_check.elapsed() >= heartbeat_check_interval {
+                let timeout = Duration::from_secs(self.config.zmq.heartbeat_timeout_seconds);
+                
+                // Clean up metrics for offline hosts
+                self.metric_store.cleanup_offline_hosts(timeout);
+                
                 if let Some(ref mut tui_app) = self.tui_app {
-                    let connected_hosts = self
-                        .metric_store
-                        .get_connected_hosts(Duration::from_secs(self.config.zmq.heartbeat_timeout_seconds));
-                    
+                    let connected_hosts = self.metric_store.get_connected_hosts(timeout);
                     tui_app.update_hosts(connected_hosts);
                 }
                 last_heartbeat_check = Instant::now();