refactor(ui/shared): scroll-lock, escape, aria-live, responsive sidebar

ConfirmDialog locks body scroll via IJSRuntime + Bootstrap's
modal-open class on show, restores on hide. Escape key now closes
the dialog; default ConfirmButtonClass flipped from btn-danger to
btn-primary so non-destructive confirms aren't red. Destructive
callsites (Delete, Discard) get explicit ConfirmButtonClass="btn-danger".

ToastNotification adds aria-live="polite" + aria-atomic="true" on the
container and an optional autoDismissMs parameter on every Show* method.

LoadingSpinner text-muted -> text-secondary for contrast.

DataTable gains a clear (x) button on the search input and applies
disabled / aria-disabled directly to the pagination buttons.

NewFolderDialog splits backdrop and modal markup to match ConfirmDialog.

NavMenu wraps the nav list in an overflow-y scroll container so the
username/sign-out footer stays anchored, and section headers convert
from <li> to <div role="presentation">.

MainLayout adds a hamburger toggle for <lg viewports; sidebar collapses
via Bootstrap collapse data attributes.

App.razor extracts inline <style> block to a shared site.css; adds a
left-border accent on the active nav link; switches the reconnect
modal to modal-dialog-centered.

Login uses d-flex / min-vh-100 centering. NotAuthorizedView gets the
same centered layout plus the ScadaLink brand heading.

Sites.razor: only the new ConfirmButtonClass="btn-danger" follow-up.

src/ScadaLink.CentralUI/Components/Pages/Admin/Sites.razor

@@ -41,7 +41,7 @@
     </div>
 
     <ToastNotification @ref="_toast" />
-    <ConfirmDialog @ref="_confirmDialog" />
+    <ConfirmDialog @ref="_confirmDialog" ConfirmButtonClass="btn-danger" />
 
     @if (_loading)
     {