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.Host/Health/AkkaClusterNodeProvider.cs

@@ -0,0 +1,62 @@
+using Akka.Actor;
+using Akka.Cluster;
+using ScadaLink.Commons.Messages.Health;
+using ScadaLink.HealthMonitoring;
+using ScadaLink.Host.Actors;
+
+namespace ScadaLink.Host.Health;
+
+/// <summary>
+/// Provides cluster node statuses from Akka.NET cluster membership for health reporting.
+/// </summary>
+public class AkkaClusterNodeProvider : IClusterNodeProvider
+{
+    private readonly AkkaHostedService _akkaService;
+    private readonly string _siteRole;
+
+    public AkkaClusterNodeProvider(AkkaHostedService akkaService, string siteRole)
+    {
+        _akkaService = akkaService;
+        _siteRole = siteRole;
+    }
+
+    public IReadOnlyList<NodeStatus> GetClusterNodes()
+    {
+        var system = _akkaService.ActorSystem;
+        if (system == null) return [];
+
+        var cluster = Cluster.Get(system);
+        var selfAddress = cluster.SelfAddress;
+        var leader = cluster.State.Leader;
+
+        var nodes = new List<NodeStatus>();
+        foreach (var member in cluster.State.Members)
+        {
+            if (!member.HasRole(_siteRole))
+                continue;
+
+            var hostname = member.Address.Host ?? member.Address.ToString();
+            var isOnline = member.Status == MemberStatus.Up;
+            var isLeader = member.Address.Equals(leader);
+            var role = isLeader ? "Primary" : "Standby";
+
+            nodes.Add(new NodeStatus(hostname, isOnline, role));
+        }
+
+        // If we have unreachable members, add them as offline
+        foreach (var unreachable in cluster.State.Unreachable)
+        {
+            if (!unreachable.HasRole(_siteRole))
+                continue;
+
+            // Don't duplicate if already in members list
+            if (nodes.Any(n => n.Hostname == (unreachable.Address.Host ?? unreachable.Address.ToString())))
+                continue;
+
+            var hostname = unreachable.Address.Host ?? unreachable.Address.ToString();
+            nodes.Add(new NodeStatus(hostname, false, "Standby"));
+        }
+
+        return nodes;
+    }
+}