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/AgentWorker.cs

@@ -6,7 +6,7 @@ namespace ITNexusAgent;
 
 public class AgentWorker
 {
-    private const string Version = "2.0.0";
+    private const string Version = "2.1.2";
     private const string DataDir = @"C:\ProgramData\IT Nexus Agent";
     private const string ConfigPath = @"C:\ProgramData\IT Nexus Agent\config.json";
     private const string StatusPath = @"C:\ProgramData\IT Nexus Agent\status.json";
@@ -39,6 +39,10 @@ public class AgentWorker
 
         Log($"Agent v{Version} gestartet");
 
+        // WebSocket Shell-Service im Hintergrund starten
+        var shellService = new ShellService(_config.ServerUrl, _config.AgentKey, SystemInfoService.GetHostname());
+        _ = shellService.RunAsync(_ct);
+
         while (!_ct.IsCancellationRequested)
         {
             await RunCycleAsync();
@@ -79,12 +83,16 @@ public class AgentWorker
                 await executor.ExecuteAsync(cmd);
             }
 
-            // Ankündigungen anzeigen
+            // Ankündigungen anzeigen + sofort ACKen damit Server sie nicht mehr schickt
             if (response.Announcements.Count > 0)
             {
                 _notifier!.ShowAnnouncements(response.Announcements);
                 foreach (var ann in response.Announcements)
+                {
                     Log($"ANNOUNCEMENT: Dialog für {SystemInfoService.GetLastUser()} gestartet - ID {ann.Id}");
+                    try { await _api!.AckAnnouncementAsync(ann.Id, payload.Hostname); }
+                    catch (Exception ex) { Log($"ANNOUNCEMENT ACK Fehler: {ex.Message}"); }
+                }
             }
         }
         catch (Exception ex)