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/Layout/MainLayout.razor

@@ -1,8 +1,23 @@
 @inherits LayoutComponentBase
 
-<div class="d-flex">
-    <NavMenu />
-    <main class="flex-grow-1 p-3" style="min-height: 100vh; background-color: #f8f9fa;">
+<div class="d-flex flex-column flex-lg-row" style="min-height: 100vh;">
+    @* Hamburger toggle: visible only on viewports <lg.
+       Bootstrap collapse JS lives in bootstrap.bundle.min.js (loaded in App.razor). *@
+    <button class="btn btn-outline-secondary btn-sm d-lg-none m-2 align-self-start"
+            type="button"
+            data-bs-toggle="collapse"
+            data-bs-target="#sidebar-collapse"
+            aria-controls="sidebar-collapse"
+            aria-expanded="false"
+            aria-label="Toggle navigation">
+        &#9776;
+    </button>
+
+    <div class="collapse d-lg-block" id="sidebar-collapse">
+        <NavMenu />
+    </div>
+
+    <main class="flex-grow-1 p-3" style="background-color: #f8f9fa;">
         @Body
     </main>
 </div>