dashboard: lazy-load BrowsePage via DashboardBrowseService

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardBrowseModel.cs

@@ -29,6 +29,33 @@ public sealed class DashboardBrowseNode
 
     /// <summary>True when the node has child objects or attributes to expand.</summary>
     public bool HasChildren => Children.Count > 0 || Object.Attributes.Count > 0;
+
+    /// <summary>Whether this node has at least one matching descendant, per the
+    /// server's <c>child_has_children</c> projector hint. Controls whether the UI
+    /// shows an expand triangle before children have actually loaded.</summary>
+    public bool HasChildrenHint { get; init; }
+
+    /// <summary>The lazy-load state for this node's children.</summary>
+    public BrowseLoadState LoadState { get; set; } = BrowseLoadState.NotLoaded;
+
+    /// <summary>Short error string if the last load attempt failed; null otherwise.</summary>
+    public string? LoadError { get; set; }
+}
+
+/// <summary>Lazy-load lifecycle of a browse node's children.</summary>
+public enum BrowseLoadState
+{
+    /// <summary>Children have not been requested yet.</summary>
+    NotLoaded,
+
+    /// <summary>A load is in progress.</summary>
+    Loading,
+
+    /// <summary>Children have been loaded into <see cref="DashboardBrowseNode.Children"/>.</summary>
+    Loaded,
+
+    /// <summary>The last load attempt failed; see <see cref="DashboardBrowseNode.LoadError"/>.</summary>
+    Error,
 }
 
 /// <summary>