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/Design/Templates.razor

@@ -10,10 +10,10 @@
 @inject TemplateFolderService TemplateFolderService
 @inject AuthenticationStateProvider AuthStateProvider
 @inject NavigationManager NavigationManager
+@inject IDialogService Dialog
 
 <div class="container-fluid mt-3">
     <ToastNotification @ref="_toast" />
-    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
 
     <RenameFolderDialog @bind-IsVisible="_showRenameFolderDialog"
                         FolderId="_renameFolderId"
@@ -21,11 +21,6 @@
                         ErrorMessage="@_renameFolderError"
                         OnSubmit="SubmitRenameFolder" />
 
-    <NewFolderDialog @bind-IsVisible="_showNewFolderDialog"
-                     ParentFolderId="_newFolderParentId"
-                     ErrorMessage="@_newFolderError"
-                     OnSubmit="SubmitNewFolder" />
-
     <MoveTemplateDialog @bind-IsVisible="_showMoveTemplateDialog"
                         TemplateId="_moveTemplateId"
                         TemplateName="@_moveTemplateName"
@@ -112,7 +107,6 @@
     private string? _errorMessage;
 
     private ToastNotification _toast = default!;
-    private ConfirmDialog _confirmDialog = default!;
 
     protected override async Task OnInitializedAsync()
     {
@@ -345,32 +339,24 @@
         }
     };
 
-    // New-folder dialog state
-    private bool _showNewFolderDialog;
-    private int? _newFolderParentId;
-    private string? _newFolderError;
-
-    private void OpenNewFolderDialog(int? parentFolderId)
+    // New-folder dialog: replaced the dedicated <NewFolderDialog> component with
+    // IDialogService.PromptAsync. Validation failures surface via toast instead of
+    // inline error text — the prompt UI doesn't have a slot for an error message.
+    private async Task OpenNewFolderDialog(int? parentFolderId)
     {
-        _newFolderParentId = parentFolderId;
-        _newFolderError = null;
-        _showNewFolderDialog = true;
-    }
+        var name = await Dialog.PromptAsync("New folder", "Folder name", placeholder: "Folder name");
+        if (string.IsNullOrWhiteSpace(name)) return;
 
-    private async Task SubmitNewFolder((int? ParentFolderId, string Name) req)
-    {
-        _newFolderError = null;
         var user = await GetCurrentUserAsync();
-        var result = await TemplateFolderService.CreateFolderAsync(req.Name, req.ParentFolderId, user);
+        var result = await TemplateFolderService.CreateFolderAsync(name.Trim(), parentFolderId, user);
         if (result.IsSuccess)
         {
-            _showNewFolderDialog = false;
             _toast.ShowSuccess($"Folder '{result.Value.Name}' created.");
             await LoadTemplatesAsync();
         }
         else
         {
-            _newFolderError = result.Error;
+            _toast.ShowError(result.Error);
         }
     }
 
@@ -497,7 +483,7 @@
 
     private async Task DeleteFolder(int folderId, string label)
     {
-        var confirmed = await _confirmDialog.ShowAsync($"Delete folder '{label}'?", "Delete Folder");
+        var confirmed = await Dialog.ConfirmAsync("Delete Folder", $"Delete folder '{label}'?", danger: true);
         if (!confirmed) return;
 
         var user = await GetCurrentUserAsync();
@@ -515,9 +501,10 @@
 
     private async Task DeleteTemplate(Template template)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Delete Template",
             $"Delete template '{template.Name}'? This will fail if instances or child templates reference it.",
-            "Delete Template");
+            danger: true);
         if (!confirmed) return;
 
         try