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/Shared/TreeView.razor

@@ -127,7 +127,10 @@ else
                 var keys = System.Text.Json.JsonSerializer.Deserialize<List<string>>(json);
                 if (keys != null)
                 {
-                    _expandedKeys = new HashSet<string>(keys);
+                    // Union (don't replace): callers may have invoked RevealNode before
+                    // this async storage load completed. Preserving those reveal-added
+                    // keys ensures deep-link reveal isn't clobbered by the restore.
+                    foreach (var k in keys) _expandedKeys.Add(k);
                     _initialExpansionApplied = true;
                 }
             }