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>

agent-cs/config.template.json

@@ -1,4 +1,4 @@
 {
   "server_url": "https://it-nexus.cereda-systems.de",
-  "agent_key": "HIER-DEN-AGENT-KEY-EINTRAGEN"
+  "agent_key": "itx-4CPJPTHmCfdrL9D62WacCATEvuvULXcp7ECMpSaNUjsS344F6_L4Ug"
 }