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

Cargo.lock

@@ -270,7 +270,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.55"
+version = "0.1.56"
 dependencies = [
  "anyhow",
  "chrono",
@@ -292,7 +292,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.55"
+version = "0.1.56"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -315,7 +315,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.55"
+version = "0.1.56"
 dependencies = [
  "chrono",
  "serde",