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/metrics/store.rs

@@ -109,6 +109,28 @@ impl MetricStore {
             .collect()
     }
 
+    /// Clean up data for offline hosts
+    pub fn cleanup_offline_hosts(&mut self, timeout: Duration) {
+        let now = Instant::now();
+        let mut hosts_to_cleanup = Vec::new();
+
+        // Find hosts that are offline (no recent heartbeat)
+        for (hostname, &last_heartbeat) in &self.last_heartbeat {
+            if now.duration_since(last_heartbeat) > timeout {
+                hosts_to_cleanup.push(hostname.clone());
+            }
+        }
+
+        // Clear metrics for offline hosts
+        for hostname in hosts_to_cleanup {
+            if let Some(metrics) = self.current_metrics.remove(&hostname) {
+                info!("Cleared {} metrics for offline host: {}", metrics.len(), hostname);
+            }
+            // Keep heartbeat timestamp for reconnection detection
+            // Don't remove from last_heartbeat to track when host was last seen
+        }
+    }
+
     /// Cleanup old data and enforce limits
     fn cleanup_host_data(&mut self, hostname: &str) {
         let now = Instant::now();