refactor(ui/shared): introduce IDialogService + DialogHost

Eliminates the per-page <ConfirmDialog @ref="_confirmDialog"
ConfirmButtonClass="btn-danger" /> boilerplate. Pages now inject
IDialogService and call ConfirmAsync(title, message, danger: true)
programmatically.

New scoped service holds a single active dialog (throws on nested
calls), with a global DialogHost mounted once in MainLayout that
renders the modal markup, owns body scroll-lock via Bootstrap's
modal-open class, traps focus on the modal element, and handles
Escape-to-cancel.

Same service also exposes PromptAsync, used to replace the bespoke
NewFolderDialog. Both ConfirmDialog and NewFolderDialog components
are deleted — their callers (~13 pages across Admin/Design/Deployment
/Monitoring) now go through the service.

DiffDialog stays as-is — different use case (before/after content).

bUnit tests in TopologyPageTests, DataConnectionsPageTests, and
TemplatesPageTests register IDialogService in their service
collection.

Also: a top-of-file Razor comment on Sites.razor pointing future
implementers at it as the reference list-page pattern.

src/ScadaLink.CentralUI/Components/Pages/Deployment/Topology.razor

@@ -19,11 +19,11 @@
 @inject AuthenticationStateProvider AuthStateProvider
 @inject NavigationManager NavigationManager
 @inject IJSRuntime JSRuntime
+@inject IDialogService Dialog
 @implements IDisposable
 
 <div class="container-fluid mt-3">
     <ToastNotification @ref="_toast" />
-    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
 
     <CreateAreaDialog @bind-IsVisible="_showCreateAreaDialog"
                       RequireSitePicker="_createAreaRequireSitePicker"
@@ -127,7 +127,6 @@
     private string _searchText = string.Empty;
 
     private ToastNotification _toast = default!;
-    private ConfirmDialog _confirmDialog = default!;
     private DiffDialog _diffDialog = default!;
 
     // ---- Live updates ----
@@ -669,9 +668,10 @@
     // ---- Area & instance deletion ----
     private async Task DeleteArea(TopoNode node)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Delete Area",
             $"Delete area '{node.Label}'? This will fail if it has sub-areas or assigned instances.",
-            "Delete Area");
+            danger: true);
         if (!confirmed) return;
 
         _actionInProgress = true;
@@ -691,9 +691,10 @@
 
     private async Task DeleteInstance(Instance inst)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Delete Instance",
             $"Delete instance '{inst.UniqueName}'? This will remove it from the site. Store-and-forward messages will NOT be cleared.",
-            "Delete Instance");
+            danger: true);
         if (!confirmed) return;
 
         _actionInProgress = true;
@@ -745,9 +746,10 @@
 
     private async Task DisableInstance(Instance inst)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Disable Instance",
             $"Disable instance '{inst.UniqueName}'? The instance actor will be stopped.",
-            "Disable Instance");
+            danger: true);
         if (!confirmed) return;
 
         _actionInProgress = true;