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)

src/ScadaLink.CentralUI/Components/Layout/NavMenu.razor

@@ -21,7 +21,7 @@
                             <NavLink class="nav-link" href="/admin/sites">Sites</NavLink>
                         </li>
                         <li class="nav-item">
-                            <NavLink class="nav-link" href="/admin/data-connections">Data Connections</NavLink>
+                            <NavLink class="nav-link" href="/admin/connections">Connections</NavLink>
                         </li>
                         <li class="nav-item">
                             <NavLink class="nav-link" href="/admin/api-keys">API Keys</NavLink>