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/ServiceCollectionExtensions.cs

@@ -1,6 +1,7 @@
 using Microsoft.AspNetCore.Components.Authorization;
 using Microsoft.Extensions.DependencyInjection;
 using ScadaLink.CentralUI.Auth;
+using ScadaLink.CentralUI.Components.Shared;
 
 namespace ScadaLink.CentralUI;
 
@@ -16,6 +17,11 @@ public static class ServiceCollectionExtensions
         services.AddScoped<AuthenticationStateProvider, CookieAuthenticationStateProvider>();
         services.AddCascadingAuthenticationState();
 
+        // Centralised dialog service: pages inject IDialogService and a single
+        // <DialogHost /> in MainLayout renders the active dialog. See
+        // Components/Shared/IDialogService.cs.
+        services.AddScoped<IDialogService, DialogService>();
+
         return services;
     }
 }