Add: Agent v2.1.2, WebSocket Live Shell, WebRTC Remote Desktop (Beta), Announcement Push

- Agent v2.1.2: WebSocket ShellService, ACK nach WS-Ankündigungen, shown_announcements Fix
- Backend: shellServer.js mit WebSocket-Server (Shell + Announcement Push + RTC Signaling)
- Backend: patch.controller.js Fix (command_id=0 Falsy-Bug beim Auto-Update)
- Frontend: Remote Desktop Tab (WebRTC Beta) in AgentDetailPage für super_admin/admin
- Frontend: PatchManagementPage auf v2.1.2 aktualisiert
- WPF Notification: AllowsTransparency=False + kein DropShadowEffect (Dispatcher-Crash Fix)

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

frontend/nginx.conf

@@ -93,6 +93,18 @@ server {
         proxy_send_timeout 300s;
     }
 
+    # WebSocket Proxy (Remote Shell + Agent-Verbindungen)
+    location /ws {
+        proxy_pass http://backend:5000/ws;
+        proxy_http_version 1.1;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_read_timeout 3600s;
+        proxy_send_timeout 3600s;
+    }
+
     # API Proxy zum Backend
     location /api/ {
         proxy_pass http://backend:5000/api/;