refactor(uns): modal-polish nits on the global UNS page (task #137)

Low-severity review nits, no behaviour change to the happy path:
- CloseModals() now also resets the leftover _*ModalIsNew / parent-id fields
  (area ClusterId, line AreaId, equipment LineId, tag/vtag) for symmetry —
  harmless today (always set before a modal opens) but consistent.
- HandleAddChild / HandleAddVirtualTag / HandleEdit gain a _modalBusy guard
  (try/finally) so a rapid double-action can't race two service loads into the
  same modal state. The switch bodies are re-indented under the try block.
- VirtualTagModal DataType is now an InputSelect over the standard OPC UA type
  list (the same set TagModal uses) instead of free-text InputText.
- RefreshEquipmentChildrenAsync documents that callers own StateHasChanged()
  and the full-reload fallback is spelled out as a block with a comment.

Build clean; AdminUI.Tests 216/216.

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Components/Shared/Uns/VirtualTagModal.razor

@@ -38,8 +38,12 @@
                         <div class="row">
                             <div class="col-md-6 mb-3">
                                 <label class="form-label" for="vtag-dtype">DataType</label>
-                                <InputText id="vtag-dtype" @bind-Value="_form.DataType" class="form-control form-control-sm mono"
-                                           placeholder="Double" />
+                                <InputSelect id="vtag-dtype" @bind-Value="_form.DataType" class="form-select form-select-sm">
+                                    @foreach (var dt in DataTypes)
+                                    {
+                                        <option value="@dt">@dt</option>
+                                    }
+                                </InputSelect>
                             </div>
                             <div class="col-md-6 mb-3">
                                 <label class="form-label" for="vtag-script">Script</label>
@@ -101,6 +105,11 @@
 }
 
 @code {
+    /// <summary>The OPC UA data types offered for a virtual tag — the same set the TagModal uses.</summary>
+    private static readonly string[] DataTypes =
+        ["Boolean", "SByte", "Byte", "Int16", "UInt16", "Int32", "UInt32",
+         "Int64", "UInt64", "Float", "Double", "String", "DateTime", "Guid", "ByteString"];
+
     /// <summary>Whether the modal is shown. The host owns this flag.</summary>
     [Parameter] public bool Visible { get; set; }