feat(uns): IUnsTreeService structural load + DI registration

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Uns/IUnsTreeService.cs

@@ -0,0 +1,19 @@
+namespace ZB.MOM.WW.OtOpcUa.AdminUI.Uns;
+
+/// <summary>
+/// Loads the structural portion of the unified-namespace (UNS) browse tree —
+/// Enterprise → Cluster → Area → Line → Equipment — from the config database.
+/// Equipment children (tags/virtual tags) are summarised by count only and loaded
+/// lazily by the renderer; this service never returns Tag/VirtualTag leaf nodes.
+/// </summary>
+public interface IUnsTreeService
+{
+    /// <summary>
+    /// Loads the full structural tree. Empty clusters are retained so they remain
+    /// visible and editable. The returned nodes are detached view-models, safe to
+    /// hold and mutate UI state on after the underlying context is disposed.
+    /// </summary>
+    /// <param name="ct">A token to cancel the load.</param>
+    /// <returns>The enterprise root nodes, each populated down to equipment.</returns>
+    Task<IReadOnlyList<UnsNode>> LoadStructureAsync(CancellationToken ct = default);
+}