fix(ui/treeview): union sessionStorage keys instead of overwriting

The previous fix tried to defer page-side RevealNode to the second
render so TreeView's async sessionStorage load could finish first. In
practice Blazor Server didn't always fire a second OnAfterRenderAsync
on the page after the deep-link load, so the reveal never ran.

Real fix: change TreeView's storage-load to UNION the restored keys
with whatever's already in _expandedKeys, instead of REPLACING. That
way the page can call RevealNode whenever it wants and the storage
restore can't clobber the reveal regardless of completion order. The
page-side guard simplifies back to a one-shot reveal on first render.

Semantic note: if a deep-link reveal expands an ancestor that the user
had previously collapsed, the deep link wins. Intentional — the URL
expresses the navigation intent.

src/ScadaLink.CentralUI/Components/Pages/Design/Templates.razor

@@ -181,9 +181,7 @@
 
     protected override async Task OnAfterRenderAsync(bool firstRender)
     {
-        // Wait until second render so TreeView has finished restoring its sessionStorage
-        // expansion state; otherwise that restore races with RevealNode and overwrites it.
-        if (!_revealApplied && !firstRender && TemplateIdParam > 0 && _tree != null)
+        if (!_revealApplied && TemplateIdParam > 0 && _tree != null)
         {
             _revealApplied = true;
             await _tree.RevealNode($"t:{TemplateIdParam}", select: true);