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/Admin/DataConnections.razor

@@ -6,6 +6,7 @@
 @attribute [Authorize(Policy = AuthorizationPolicies.RequireAdmin)]
 @inject ISiteRepository SiteRepository
 @inject NavigationManager NavigationManager
+@inject IDialogService Dialog
 
 <div class="container-fluid mt-3">
     <div class="d-flex justify-content-between align-items-center mb-3">
@@ -36,7 +37,6 @@
     </div>
 
     <ToastNotification @ref="_toast" />
-    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
 
     @if (_loading)
     {
@@ -183,7 +183,6 @@
     private HashSet<string> _matchKeys = new();
 
     private ToastNotification _toast = default!;
-    private ConfirmDialog _confirmDialog = default!;
 
     private bool HasSiteSelected => ResolveSelectedSiteId() != null;
 
@@ -293,8 +292,10 @@
 
     private async Task DeleteConnection(DataConnection conn)
     {
-        var confirmed = await _confirmDialog.ShowAsync(
-            $"Delete data connection '{conn.Name}'?", "Delete Connection");
+        var confirmed = await Dialog.ConfirmAsync(
+            "Delete Connection",
+            $"Delete data connection '{conn.Name}'?",
+            danger: true);
         if (!confirmed) return;
 
         try