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/Admin/LdapMappingForm.razor

@@ -8,6 +8,7 @@
 @inject ISecurityRepository SecurityRepository
 @inject ISiteRepository SiteRepository
 @inject NavigationManager NavigationManager
+@inject IDialogService Dialog
 
 <div class="container-fluid mt-3">
     <div class="mb-3">
@@ -18,8 +19,6 @@
         </button>
     </div>
 
-    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
-
     <div class="card mb-3">
         <div class="card-body">
             <h5 class="card-title">Mapping</h5>
@@ -120,8 +119,6 @@
     private int _scopeRuleSiteId;
     private string? _scopeRuleError;
 
-    private ConfirmDialog _confirmDialog = default!;
-
     protected override async Task OnInitializedAsync()
     {
         _sites = (await SiteRepository.GetAllSitesAsync()).ToList();
@@ -209,9 +206,10 @@
     private async Task DeleteScopeRule(SiteScopeRule rule)
     {
         var siteName = _siteLookup.GetValueOrDefault(rule.SiteId)?.Name ?? $"Site {rule.SiteId}";
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Remove Scope Rule",
             $"Remove scope rule for '{siteName}'?",
-            "Remove Scope Rule");
+            danger: true);
         if (!confirmed) return;
 
         try