fix(ui/templates): stop drop propagation on folder nodes

Without stopPropagation, dropping a template onto a folder fires both
OnDrop(folder) and OnDropOnRoot via event bubbling. The two async handlers
race on the same scoped DbContext, which is not thread-safe — the second
throws ObjectDisposedException and tears down the Blazor circuit. Surfaced
during browser smoke testing via JS-dispatched DragEvent sequence.

src/ScadaLink.CentralUI/Components/Pages/Design/Templates.razor

@@ -534,7 +534,8 @@
              @ondragenter="() => OnDragEnter(node)"
              @ondragleave="() => OnDragLeave(node)"
              @ondragover:preventDefault="@isDropTarget"
-             @ondrop="() => OnDrop(node)">
+             @ondrop="() => OnDrop(node)"
+             @ondrop:stopPropagation="true">
             @switch (node.Kind)
             {
                 case TmplNodeKind.Folder: