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.

tests/ScadaLink.CentralUI.Tests/DataConnectionsPageTests.cs

@@ -3,6 +3,7 @@ using Bunit;
 using Microsoft.AspNetCore.Components.Authorization;
 using Microsoft.Extensions.DependencyInjection;
 using NSubstitute;
+using ScadaLink.CentralUI.Components.Shared;
 using ScadaLink.Commons.Entities.Sites;
 using ScadaLink.Commons.Interfaces.Repositories;
 using DataConnectionsPage = ScadaLink.CentralUI.Components.Pages.Admin.DataConnections;
@@ -21,6 +22,9 @@ public class DataConnectionsPageTests : BunitContext
     public DataConnectionsPageTests()
     {
         Services.AddSingleton(_siteRepo);
+        // Satisfy the page's [Inject] IDialogService — the host that actually
+        // renders the dialog lives in MainLayout, not in bUnit's render scope.
+        Services.AddScoped<IDialogService, DialogService>();
         AddTestAuth();
 
         JSInterop.Setup<string?>("treeviewStorage.load", _ => true).SetResult(null);