feat(centralui): collapsible sidebar nav sections

Make the seven sidebar section groups (Admin, Design, Deployment,
Notifications, Site Calls, Monitoring, Audit) collapsible. New NavSection
component renders a header toggle button (chevron) and reveals its items
only while expanded; NavMenu owns the expanded-section set.

Behaviour: sections are collapsed by default; state persists in the
`scadabridge_nav` cookie (written/read via the new nav-state.js JS interop,
mirroring treeview-storage.js) so it survives reloads and reconnects;
navigating into a section auto-expands it and remembers it. The Dashboard
item stays sectionless and always visible.

Tests: NavMenu bUnit tests expand sections before asserting items and add
collapsed-by-default / toggle / cookie-persistence cases; Playwright nav
tests expand sections before clicking links; new NavCollapseTests covers
the feature E2E. Build 0 warnings; bUnit 545 passed; Playwright nav suite
green (the unrelated AuditGridColumnTests resize-reload case remains
pre-existing flaky — an un-awaited save race in that test).

tests/ScadaLink.CentralUI.PlaywrightTests/PlaywrightFixture.cs

@@ -105,6 +105,28 @@ public class PlaywrightFixture : IAsyncLifetime
             : $"() => window.location.pathname.includes('{path}')";
         await page.WaitForFunctionAsync(js, null, new() { Timeout = timeoutMs });
     }
+
+    /// <summary>
+    /// Expand every collapsed sidebar nav section. Nav sections are collapsed by
+    /// default, so a section's links are not in the DOM until it is expanded.
+    /// Call this after authenticating, before interacting with sectioned nav links.
+    /// </summary>
+    public static async Task ExpandAllNavSectionsAsync(IPage page)
+    {
+        var toggles = page.Locator("button.nav-section-toggle");
+        int count = await toggles.CountAsync();
+        for (int i = 0; i < count; i++)
+        {
+            var toggle = toggles.Nth(i);
+            if (await toggle.GetAttributeAsync("aria-expanded") == "false")
+            {
+                await toggle.ClickAsync();
+                // Wait for the toggle's own state to flip so the Blazor
+                // re-render has landed before moving to the next section.
+                await Assertions.Expect(toggle).ToHaveAttributeAsync("aria-expanded", "true");
+            }
+        }
+    }
 }
 
 [CollectionDefinition("Playwright")]