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

Cargo.lock

@@ -270,7 +270,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.61"
+version = "0.1.62"
 dependencies = [
  "anyhow",
  "chrono",
@@ -292,7 +292,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.61"
+version = "0.1.62"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -315,7 +315,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.61"
+version = "0.1.62"
 dependencies = [
  "chrono",
  "serde",