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/Shared/NewFolderDialog.razor

@@ -1,7 +1,8 @@
 @if (IsVisible)
 {
-    <div class="modal show d-block" tabindex="-1" style="background: rgba(0,0,0,0.4);">
-        <div class="modal-dialog modal-sm">
+    <div class="modal-backdrop fade show"></div>
+    <div class="modal fade show d-block" tabindex="-1" role="dialog">
+        <div class="modal-dialog modal-dialog-centered modal-sm" role="document">
             <div class="modal-content">
                 <div class="modal-header">
                     <h6 class="modal-title">New Folder</h6>