Add RDP user consent popup before screen sharing

When rdp_start arrives, agent first spawns a WPF consent dialog in the
user session (via schtasks). User has 30s to accept or deny. On deny,
agent sends rdp_denied to browser which shows "Zugriff abgelehnt".
On accept, screen capture starts as before.

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

frontend/src/components/common/RemoteDesktopPanel.jsx

@@ -55,6 +55,10 @@ export default function RemoteDesktopPanel({ agentId, agentHostname, autoConnect
                         }
                     };
                     img.src = 'data:image/jpeg;base64,' + msg.data;
+                } else if (msg.type === 'rdp_denied') {
+                    setStatus('error');
+                    setError('Zugriff wurde vom Benutzer abgelehnt');
+                    ws.close();
                 } else if (msg.type === 'rdp_disconnected') {
                     setStatus('idle');
                     setError('Agent hat die Verbindung getrennt');