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/NavCollapseTests.cs

@@ -0,0 +1,88 @@
+using Microsoft.Playwright;
+
+namespace ScadaLink.CentralUI.PlaywrightTests;
+
+/// <summary>
+/// E2E tests for the collapsible sidebar nav sections: sections are collapsed
+/// by default, a header toggle reveals a section's items, the state persists in
+/// the <c>scadabridge_nav</c> cookie across a full page reload, and navigating
+/// into a section auto-expands it.
+/// </summary>
+[Collection("Playwright")]
+public class NavCollapseTests
+{
+    private readonly PlaywrightFixture _fixture;
+
+    public NavCollapseTests(PlaywrightFixture fixture)
+    {
+        _fixture = fixture;
+    }
+
+    [Fact]
+    public async Task Sections_AreCollapsedByDefault_AfterLogin()
+    {
+        var page = await _fixture.NewAuthenticatedPageAsync();
+
+        // The dashboard is sectionless, so no section is auto-expanded and the
+        // cookie is empty on a fresh context — every section toggle is collapsed.
+        await Expect(page.Locator("button.nav-section-toggle[aria-expanded='true']"))
+            .ToHaveCountAsync(0);
+        // A sectioned link is therefore absent from the DOM.
+        Assert.Equal(0, await page.Locator("nav a:has-text('Topology')").CountAsync());
+    }
+
+    [Fact]
+    public async Task ClickingSectionHeader_RevealsItsItems()
+    {
+        var page = await _fixture.NewAuthenticatedPageAsync();
+        var toggle = page.Locator("button.nav-section-toggle:has-text('Deployment')");
+
+        Assert.Equal(0, await page.Locator("nav a:has-text('Topology')").CountAsync());
+
+        await toggle.ClickAsync();
+
+        await Expect(toggle).ToHaveAttributeAsync("aria-expanded", "true");
+        await Expect(page.GetByRole(AriaRole.Link, new() { Name = "Topology" }))
+            .ToBeVisibleAsync();
+    }
+
+    [Fact]
+    public async Task CollapseState_SurvivesPageReload()
+    {
+        var page = await _fixture.NewAuthenticatedPageAsync();
+        await page.Locator("button.nav-section-toggle:has-text('Deployment')").ClickAsync();
+        await Expect(page.GetByRole(AriaRole.Link, new() { Name = "Topology" }))
+            .ToBeVisibleAsync();
+
+        await page.ReloadAsync();
+        await page.WaitForLoadStateAsync(LoadState.NetworkIdle);
+
+        // The scadabridge_nav cookie restored the expanded Deployment section.
+        await Expect(page.Locator("button.nav-section-toggle:has-text('Deployment')"))
+            .ToHaveAttributeAsync("aria-expanded", "true");
+        await Expect(page.GetByRole(AriaRole.Link, new() { Name = "Topology" }))
+            .ToBeVisibleAsync();
+    }
+
+    [Fact]
+    public async Task NavigatingIntoCollapsedSection_AutoExpandsIt()
+    {
+        var page = await _fixture.NewAuthenticatedPageAsync();
+        var auditToggle = page.Locator("button.nav-section-toggle:has-text('Audit')");
+
+        // The Audit section starts collapsed.
+        await Expect(auditToggle).ToHaveAttributeAsync("aria-expanded", "false");
+
+        // Navigate into the Audit section via an in-page link (SPA navigation,
+        // which raises NavigationManager.LocationChanged) — the Configuration
+        // Audit Log quick-action card on the dashboard.
+        await page.Locator("a[href='/audit/configuration']").First.ClickAsync();
+        await PlaywrightFixture.WaitForPathAsync(page, "/audit/configuration");
+
+        // The Audit nav section auto-expanded on arrival.
+        await Expect(auditToggle).ToHaveAttributeAsync("aria-expanded", "true");
+    }
+
+    private static ILocatorAssertions Expect(ILocator locator) =>
+        Assertions.Expect(locator);
+}

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);

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")]

tests/ScadaLink.CentralUI.PlaywrightTests/RoleNavigationTests.cs

@@ -220,6 +220,9 @@ public class RoleNavigationTests
 
     private static async Task AssertNavLinkVisible(IPage page, string linkText)
     {
+        // Sections are collapsed by default — expand them so a present link is
+        // in the DOM. Idempotent: already-expanded sections are skipped.
+        await PlaywrightFixture.ExpandAllNavSectionsAsync(page);
         var locator = page.Locator($"nav a:has-text('{linkText}')");
         var count = await locator.CountAsync();
         Assert.True(count > 0, $"Expected nav link '{linkText}' to be visible, but it was not found");