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.Tests/Pages/AuditLogPageScaffoldTests.cs

@@ -103,6 +103,18 @@ public class AuditLogPageScaffoldTests : BunitContext
         return host.FindComponent<NavMenu>();
     }
 
+    /// <summary>
+    /// Clicks the collapsible section header whose title matches, expanding it.
+    /// Nav sections are collapsed by default, so a section's items are only in
+    /// the DOM once expanded.
+    /// </summary>
+    private static void ExpandNavSection(IRenderedComponent<NavMenu> cut, string title)
+    {
+        var toggle = cut.FindAll("button.nav-section-toggle")
+            .Single(b => b.TextContent.Contains(title, StringComparison.Ordinal));
+        toggle.Click();
+    }
+
     [Fact]
     public void AuditLogPage_Renders_PageHeading()
     {
@@ -121,6 +133,7 @@ public class AuditLogPageScaffoldTests : BunitContext
     public void NavMenu_Contains_AuditGroup_With_AuditLog_Link()
     {
         var cut = RenderNavMenu("Admin", "Design", "Deployment");
+        ExpandNavSection(cut, "Audit");
 
         cut.WaitForAssertion(() =>
         {
@@ -133,6 +146,7 @@ public class AuditLogPageScaffoldTests : BunitContext
     public void NavMenu_Contains_ConfigurationAuditLog_Link_UnderAuditGroup()
     {
         var cut = RenderNavMenu("Admin", "Design", "Deployment");
+        ExpandNavSection(cut, "Audit");
 
         cut.WaitForAssertion(() =>
         {