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.PlaywrightTests/NavigationTests.cs

@@ -25,7 +25,7 @@ public class NavigationTests
 
     [Theory]
     [InlineData("Sites", "/admin/sites")]
-    [InlineData("Data Connections", "/admin/data-connections")]
+    [InlineData("Connections", "/admin/connections")]
     [InlineData("API Keys", "/admin/api-keys")]
     [InlineData("LDAP Mappings", "/admin/ldap-mappings")]
     public async Task AdminNavLinks_NavigateCorrectly(string linkText, string expectedPath)