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

agent/src/config/validation.rs

@@ -19,10 +19,6 @@ pub fn validate_config(config: &AgentConfig) -> Result<()> {
         bail!("ZMQ bind address cannot be empty");
     }
 
-    if config.zmq.timeout_ms == 0 {
-        bail!("ZMQ timeout cannot be 0");
-    }
-
     // Validate collection interval
     if config.collection_interval_seconds == 0 {
         bail!("Collection interval cannot be 0");