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

agent/src/config/mod.rs

@@ -17,9 +17,6 @@ pub struct AgentConfig {
     pub notifications: NotificationConfig,
     pub status_aggregation: HostStatusConfig,
     pub collection_interval_seconds: u64,
-    /// List of metric names to exclude from email notifications
-    #[serde(default)]
-    pub exclude_email_metrics: Vec<String>,
 }
 
 /// ZMQ communication configuration
@@ -29,6 +26,9 @@ pub struct ZmqConfig {
     pub command_port: u16,
     pub bind_address: String,
     pub transmission_interval_seconds: u64,
+    /// Heartbeat transmission interval in seconds for host connectivity detection
+    #[serde(default = "default_heartbeat_interval_seconds")]
+    pub heartbeat_interval_seconds: u64,
 }
 
 /// Collector configuration
@@ -147,9 +147,16 @@ pub struct NotificationConfig {
     pub rate_limit_minutes: u64,
     /// Email notification batching interval in seconds (default: 60)
     pub aggregation_interval_seconds: u64,
+    /// List of metric names to exclude from email notifications
+    #[serde(default)]
+    pub exclude_email_metrics: Vec<String>,
 }
 
 
+fn default_heartbeat_interval_seconds() -> u64 {
+    5
+}
+
 impl AgentConfig {
     pub fn from_file<P: AsRef<Path>>(path: P) -> Result<Self> {
         loader::load_config(path)