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/ConfirmDialog.razor

@@ -1,9 +1,16 @@
-@* Reusable confirmation dialog using Bootstrap modal *@
+@* Reusable confirmation dialog using Bootstrap modal.
+   z-index ladder: Toast container 1090 > this modal 1055 > this backdrop 1040. *@
+@inject IJSRuntime JS
+@implements IAsyncDisposable
 
 @if (_visible)
 {
     <div class="modal-backdrop fade show"></div>
-    <div class="modal fade show d-block" tabindex="-1" role="dialog">
+    <div @ref="_modalRef"
+         class="modal fade show d-block"
+         tabindex="-1"
+         role="dialog"
+         @onkeydown="OnKeyDownAsync">
         <div class="modal-dialog modal-dialog-centered" role="document">
             <div class="modal-content">
                 <div class="modal-header">
@@ -24,12 +31,14 @@
 
 @code {
     private bool _visible;
+    private bool _bodyLocked;
     private TaskCompletionSource<bool>? _tcs;
+    private ElementReference _modalRef;
 
     [Parameter] public string Title { get; set; } = "Confirm";
     [Parameter] public string Message { get; set; } = "Are you sure?";
     [Parameter] public string ConfirmText { get; set; } = "Confirm";
-    [Parameter] public string ConfirmButtonClass { get; set; } = "btn-danger";
+    [Parameter] public string ConfirmButtonClass { get; set; } = "btn-primary";
 
     public Task<bool> ShowAsync(string? message = null, string? title = null)
     {
@@ -41,15 +50,82 @@
         return _tcs.Task;
     }
 
+    protected override async Task OnAfterRenderAsync(bool firstRender)
+    {
+        if (_visible && !_bodyLocked)
+        {
+            _bodyLocked = true;
+            await TryLockBodyAsync();
+            // Focus the modal so the @onkeydown handler receives Escape.
+            try { await _modalRef.FocusAsync(); }
+            catch { /* prerender or detached: ignore */ }
+        }
+    }
+
+    private async Task OnKeyDownAsync(KeyboardEventArgs e)
+    {
+        if (e.Key == "Escape")
+        {
+            await CancelAsync();
+        }
+    }
+
     private void Confirm()
     {
-        _visible = false;
-        _tcs?.TrySetResult(true);
+        Close(true);
     }
 
     private void Cancel()
+    {
+        Close(false);
+    }
+
+    private Task CancelAsync()
+    {
+        Close(false);
+        return Task.CompletedTask;
+    }
+
+    private void Close(bool result)
     {
         _visible = false;
-        _tcs?.TrySetResult(false);
+        _ = TryUnlockBodyAsync();
+        _tcs?.TrySetResult(result);
+    }
+
+    private async Task TryLockBodyAsync()
+    {
+        try
+        {
+            await JS.InvokeVoidAsync("document.body.classList.add", "modal-open");
+        }
+        catch
+        {
+            // Prerendering has no JS runtime; log only.
+            try { await JS.InvokeVoidAsync("console.debug", "ConfirmDialog: JS interop unavailable for body lock."); }
+            catch { /* swallow */ }
+        }
+    }
+
+    private async Task TryUnlockBodyAsync()
+    {
+        _bodyLocked = false;
+        try
+        {
+            await JS.InvokeVoidAsync("document.body.classList.remove", "modal-open");
+        }
+        catch
+        {
+            try { await JS.InvokeVoidAsync("console.debug", "ConfirmDialog: JS interop unavailable for body unlock."); }
+            catch { /* swallow */ }
+        }
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        if (_bodyLocked)
+        {
+            await TryUnlockBodyAsync();
+        }
     }
 }