Complete OPC UA data flow: binding UI, flattening connections, real OPC UA client

- Add connection binding UI to Instances page (per-attribute and bulk assign)
- FlatteningService populates Connections dict from bound data connections
- Real OPC UA client using OPC Foundation SDK for live tag subscriptions
- DataConnectionFactory uses RealOpcUaClientFactory by default
- OpcUaDataConnection supports both "endpoint" and "EndpointUrl" config keys

src/ScadaLink.DataConnectionLayer/Adapters/OpcUaDataConnection.cs

@@ -37,7 +37,12 @@ public class OpcUaDataConnection : IDataConnection
 
     public async Task ConnectAsync(IDictionary<string, string> connectionDetails, CancellationToken cancellationToken = default)
     {
-        _endpointUrl = connectionDetails.TryGetValue("EndpointUrl", out var url) ? url : "opc.tcp://localhost:4840";
+        // Support both "endpoint" (from JSON config) and "EndpointUrl" (programmatic)
+        _endpointUrl = connectionDetails.TryGetValue("endpoint", out var url)
+            ? url
+            : connectionDetails.TryGetValue("EndpointUrl", out var url2)
+                ? url2
+                : "opc.tcp://localhost:4840";
         _status = ConnectionHealth.Connecting;
 
         _client = _clientFactory.Create();