Add Remote Desktop screen selector + new-tab button (v2.2.0)

- CaptureModeRunner: screen index param (0=all, 1-N=specific monitor)
- RtcService: reads screen from rdp_start JSON, passes to capture helper
- Program.cs: parses optional screen index arg for --rdp-capture
- RemoteDesktopPanel: screen dropdown (Alle/1-4), new-tab button
- RdpPage: fullscreen standalone page at /rdp/:agentId
- AgentDetailPage: removed dead _RemoteDesktopOld_unused code

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

agent-cs/Program.cs

@@ -20,7 +20,9 @@ internal class Program
                     return;
 
                 case "--rdp-capture":
-                    CaptureModeRunner.Run(args.Length > 1 ? args[1] : "");
+                    CaptureModeRunner.Run(
+                        args.Length > 1 ? args[1] : "",
+                        args.Length > 2 && int.TryParse(args[2], out var si) ? si : 0);
                     return;
 
                 case "--dashboard":