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/NavigationTests.cs

@@ -77,6 +77,8 @@ public class NavigationTests
 
     private static async Task ClickNavAndWait(IPage page, string linkText, string expectedPath)
     {
+        // Sections are collapsed by default — open them so the link is in the DOM.
+        await PlaywrightFixture.ExpandAllNavSectionsAsync(page);
         await page.Locator($"nav a:has-text('{linkText}')").ClickAsync();
         await PlaywrightFixture.WaitForPathAsync(page, expectedPath);
         Assert.Contains(expectedPath, page.Url);