feat(ui/admin): Topology-style refresh of Data Connections page

Brings the Data Connections admin page up to the same UX standard as the
Topology page:
- Search box with dim non-matches (opacity 0.4, shape preserved)
- Toolbar: + Connection (disabled until a site is selected), Refresh,
  Expand, Collapse
- Site context menu gains "Add Connection here" that navigates with
  ?siteId= so the form preselects + locks the Site field
- Form gains "Primary Endpoint" / "Backup Endpoint" h6 subsection
  headers matching the SiteForm convention; Failover Retry Count moved
  inside the Backup subsection
- URL renamed: /admin/connections (primary) + /admin/data-connections
  (legacy secondary @page). Same dual-route treatment on the form
- Nav label: "Data Connections" -> "Connections"
- Adds DataConnectionsPageTests bUnit suite (6 tests)

tests/ScadaLink.CentralUI.Tests/DataConnectionsPageTests.cs

@@ -0,0 +1,158 @@
+using System.Security.Claims;
+using Bunit;
+using Microsoft.AspNetCore.Components.Authorization;
+using Microsoft.Extensions.DependencyInjection;
+using NSubstitute;
+using ScadaLink.Commons.Entities.Sites;
+using ScadaLink.Commons.Interfaces.Repositories;
+using DataConnectionsPage = ScadaLink.CentralUI.Components.Pages.Admin.DataConnections;
+
+namespace ScadaLink.CentralUI.Tests;
+
+/// <summary>
+/// bUnit rendering tests for the Connections page (Site → DataConnection tree).
+/// Focuses on the Topology-style behaviors layered onto this page: always-show-empty
+/// sites, search dimming, toolbar gating, and dual-route declaration.
+/// </summary>
+public class DataConnectionsPageTests : BunitContext
+{
+    private readonly ISiteRepository _siteRepo = Substitute.For<ISiteRepository>();
+
+    public DataConnectionsPageTests()
+    {
+        Services.AddSingleton(_siteRepo);
+        AddTestAuth();
+
+        JSInterop.Setup<string?>("treeviewStorage.load", _ => true).SetResult(null);
+        JSInterop.SetupVoid("treeviewStorage.save", _ => true);
+    }
+
+    private void AddTestAuth()
+    {
+        var claims = new[]
+        {
+            new Claim("Username", "tester"),
+            new Claim(ClaimTypes.Role, "Admin")
+        };
+        var user = new ClaimsPrincipal(new ClaimsIdentity(claims, "TestAuth"));
+        Services.AddSingleton<AuthenticationStateProvider>(new TestAuthStateProvider(user));
+        Services.AddAuthorizationCore();
+    }
+
+    private void SeedRepos(
+        IEnumerable<Site>? sites = null,
+        IEnumerable<DataConnection>? connections = null)
+    {
+        _siteRepo.GetAllSitesAsync(Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<Site>>(sites?.ToList() ?? new List<Site>()));
+        _siteRepo.GetAllDataConnectionsAsync(Arg.Any<CancellationToken>())
+            .Returns(Task.FromResult<IReadOnlyList<DataConnection>>(connections?.ToList() ?? new List<DataConnection>()));
+    }
+
+    private static AngleSharp.Dom.IElement? FindToggleForLabel(IRenderedComponent<DataConnectionsPage> cut, string label) =>
+        cut.FindAll(".tv-row")
+            .FirstOrDefault(row => row.QuerySelector(".tv-label")?.TextContent == label)
+            ?.QuerySelector(".tv-toggle");
+
+    [Fact]
+    public void Renders_EmptyState_WhenNoSites()
+    {
+        SeedRepos();
+
+        var cut = Render<DataConnectionsPage>();
+
+        Assert.Contains("No sites configured", cut.Markup);
+    }
+
+    [Fact]
+    public void Renders_EmptySite_AsTopLevelNode()
+    {
+        // A site with no connections must still appear so it can be right-clicked
+        // to "Add Connection here".
+        SeedRepos(sites: new[] { new Site("Plant-A", "plant-a") { Id = 1 } });
+
+        var cut = Render<DataConnectionsPage>();
+
+        Assert.Contains("Plant-A", cut.Markup);
+    }
+
+    [Fact]
+    public void Renders_SiteConnection_Nesting()
+    {
+        SeedRepos(
+            sites: new[] { new Site("Plant-A", "plant-a") { Id = 1 } },
+            connections: new[]
+            {
+                new DataConnection("PLC-1", "OpcUa", 1) { Id = 100 }
+            });
+
+        var cut = Render<DataConnectionsPage>();
+        FindToggleForLabel(cut, "Plant-A")!.Click();
+
+        Assert.Contains("PLC-1", cut.Markup);
+        Assert.Contains("OpcUa", cut.Markup);
+    }
+
+    [Fact]
+    public void Search_DimsNonMatches_PreservesShape()
+    {
+        SeedRepos(
+            sites: new[]
+            {
+                new Site("Plant-A", "plant-a") { Id = 1 },
+                new Site("Plant-B", "plant-b") { Id = 2 }
+            },
+            connections: new[]
+            {
+                new DataConnection("PLC-1", "OpcUa", 1) { Id = 100 },
+                new DataConnection("RTU-9", "Custom", 2) { Id = 200 }
+            });
+
+        var cut = Render<DataConnectionsPage>();
+
+        var search = cut.Find("input[type='text']");
+        search.Input("Plant-A");
+
+        // Both sites remain in the DOM (shape preserved). Plant-B gets the dim style.
+        Assert.Contains("Plant-A", cut.Markup);
+        Assert.Contains("Plant-B", cut.Markup);
+        var dimmedNodes = cut.FindAll("span.tv-label[style*='opacity']");
+        Assert.Contains(dimmedNodes, n => n.TextContent.Contains("Plant-B"));
+    }
+
+    [Fact]
+    public void AddConnectionButton_DisabledUntilSiteSelected()
+    {
+        SeedRepos(sites: new[] { new Site("Plant-A", "plant-a") { Id = 1 } });
+
+        var cut = Render<DataConnectionsPage>();
+
+        var addBtn = cut.FindAll("button")
+            .First(b => b.TextContent.Contains("+ Connection"));
+        Assert.True(addBtn.HasAttribute("disabled"));
+
+        // Click the site content (TreeView wires selection on .tv-content).
+        var siteContent = cut.FindAll(".tv-row")
+            .First(r => r.QuerySelector(".tv-label")?.TextContent == "Plant-A")
+            .QuerySelector(".tv-content")!;
+        siteContent.Click();
+
+        var addBtnAfter = cut.FindAll("button")
+            .First(b => b.TextContent.Contains("+ Connection"));
+        Assert.False(addBtnAfter.HasAttribute("disabled"));
+    }
+
+    [Fact]
+    public void LegacyDataConnectionsRoute_IsDeclaredOnListPage()
+    {
+        // Old bookmarks to /admin/data-connections must still resolve.
+        var routes = typeof(DataConnectionsPage).GetCustomAttributes(
+            typeof(Microsoft.AspNetCore.Components.RouteAttribute), inherit: false)
+            .Cast<Microsoft.AspNetCore.Components.RouteAttribute>()
+            .Select(a => a.Template)
+            .ToList();
+
+        Assert.Contains("/admin/connections", routes);
+        Assert.Contains("/admin/data-connections", routes);
+    }
+}