feat(health): show all cluster nodes (online/offline, primary/standby) in health dashboard

Add NodeStatus record, IClusterNodeProvider interface, and AkkaClusterNodeProvider
that queries Akka cluster membership for all site-role nodes. HealthReportSender
populates ClusterNodes before each report. UI shows a row per node with
hostname, Online/Offline badge, and Primary/Standby badge. Falls back to
single-node display if ClusterNodes is not populated.

src/ScadaLink.HealthMonitoring/SiteHealthCollector.cs

@@ -21,6 +21,7 @@ public class SiteHealthCollector : ISiteHealthCollector
     private int _deployedInstanceCount, _enabledInstanceCount, _disabledInstanceCount;
     private int _parkedMessageCount;
     private volatile string _nodeHostname = "";
+    private volatile IReadOnlyList<Commons.Messages.Health.NodeStatus>? _clusterNodes;
     private volatile bool _isActiveNode;
 
     /// <summary>
@@ -94,6 +95,8 @@ public class SiteHealthCollector : ISiteHealthCollector
 
     public void SetNodeHostname(string hostname) => _nodeHostname = hostname;
 
+    public void SetClusterNodes(IReadOnlyList<Commons.Messages.Health.NodeStatus> nodes) => _clusterNodes = nodes;
+
     /// <summary>
     /// Set the current store-and-forward buffer depths snapshot.
     /// Called before report collection with data from the S&amp;F service.
@@ -159,6 +162,7 @@ public class SiteHealthCollector : ISiteHealthCollector
             NodeHostname: _nodeHostname,
             DataConnectionEndpoints: connectionEndpoints,
             DataConnectionTagQuality: tagQuality,
-            ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0));
+            ParkedMessageCount: Interlocked.CompareExchange(ref _parkedMessageCount, 0, 0),
+            ClusterNodes: _clusterNodes?.ToList());
     }
 }