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/TemplatesPageTests.cs

@@ -6,6 +6,7 @@ using NSubstitute;
 using ScadaLink.Commons.Entities.Templates;
 using ScadaLink.Commons.Interfaces.Repositories;
 using ScadaLink.Commons.Interfaces.Services;
+using ScadaLink.CentralUI.Components.Shared;
 using ScadaLink.TemplateEngine;
 using ScadaLink.TemplateEngine.Services;
 using TemplatesPage = ScadaLink.CentralUI.Components.Pages.Design.Templates;
@@ -30,6 +31,10 @@ public class TemplatesPageTests : BunitContext
         Services.AddSingleton(_audit);
         Services.AddScoped<TemplateService>();
         Services.AddScoped<TemplateFolderService>();
+        // The Templates page injects IDialogService for the new-folder prompt
+        // and delete confirmations. The host is rendered in MainLayout, not
+        // here, but the DI registration still has to satisfy the [Inject].
+        Services.AddScoped<IDialogService, DialogService>();
         AddTestAuth();
 
         // The TreeView inside the page persists expansion state via JS interop