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/Monitoring/ParkedMessages.razor

@@ -7,12 +7,12 @@
 @inject ISiteRepository SiteRepository
 @inject CommunicationService CommunicationService
 @inject IJSRuntime JS
+@inject IDialogService Dialog
 
 <div class="container-fluid mt-3">
     <h4 class="mb-3">Parked Messages</h4>
 
     <ToastNotification @ref="_toast" />
-    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
 
     <div class="row mb-3 g-2 align-items-end">
         <div class="col-md-3">
@@ -153,7 +153,6 @@
     private string? _activeMessageId;
     private string? _activeAction;
     private ToastNotification _toast = default!;
-    private ConfirmDialog _confirmDialog = default!;
     private readonly HashSet<int> _expandedRows = new();
 
     protected override async Task OnInitializedAsync()
@@ -259,9 +258,10 @@
 
     private async Task DiscardMessage(ParkedMessageEntry msg)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
+        var confirmed = await Dialog.ConfirmAsync(
+            "Discard Parked Message",
             $"Permanently discard message {msg.MessageId[..Math.Min(12, msg.MessageId.Length)]}? This cannot be undone.",
-            "Discard Parked Message");
+            danger: true);
         if (!confirmed) return;
 
         _actionInProgress = true;