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/Layout/MainLayout.razor

@@ -21,3 +21,7 @@
         @Body
     </main>
 </div>
+
+@* Global host for IDialogService. One instance per layout renders all confirm/prompt
+   dialogs raised via IDialogService.ConfirmAsync / PromptAsync. *@
+<DialogHost />