Implement heartbeat-based host connectivity detection

- Add agent_heartbeat metric to agent transmission for reliable host detection
- Update dashboard to track heartbeat timestamps per host instead of general metrics
- Add configurable heartbeat_timeout_seconds to dashboard ZMQ config (default 10s)
- Remove unused timeout_ms from agent config and revert to non-blocking command reception
- Remove unused heartbeat_interval_ms from agent configuration
- Host disconnect detection now uses dedicated heartbeat metrics for improved reliability
- Bump version to 0.1.57

dashboard/src/communication/mod.rs

@@ -141,9 +141,9 @@ impl ZmqConsumer {
         }
     }
 
-    /// Receive metrics from any connected agent (non-blocking)
+    /// Receive metrics from any connected agent (with timeout)
     pub async fn receive_metrics(&mut self) -> Result<Option<MetricMessage>> {
-        match self.subscriber.recv_bytes(zmq::DONTWAIT) {
+        match self.subscriber.recv_bytes(0) {
             Ok(data) => {
                 debug!("Received {} bytes from ZMQ", data.len());