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/Program.cs

@@ -39,15 +39,38 @@ internal class Program
 
     static void RunNotification(string base64Json)
     {
+        const string errorLog = @"C:\ProgramData\IT Nexus Agent\notify-error.log";
         try
         {
             var json = Encoding.UTF8.GetString(Convert.FromBase64String(base64Json));
+            File.AppendAllText(errorLog, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} START json={json}\n");
+
             var ann = JsonConvert.DeserializeObject<Announcement>(json);
-            if (ann == null) return;
+            if (ann == null)
+            {
+                File.AppendAllText(errorLog, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} ERROR: Deserialization returned null\n");
+                return;
+            }
+
             var app = new System.Windows.Application();
-            app.Run(new NotificationWindow(ann));
+            app.ShutdownMode = System.Windows.ShutdownMode.OnMainWindowClose;
+            app.DispatcherUnhandledException += (s, e) =>
+            {
+                File.AppendAllText(errorLog, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} DISPATCHER ERROR: {e.Exception}\n");
+                e.Handled = true;
+                app.Shutdown();
+            };
+
+            var win = new NotificationWindow(ann);
+            win.Topmost = true;
+            win.Show();
+            win.Activate();
+            app.Run();
+        }
+        catch (Exception ex)
+        {
+            File.AppendAllText(errorLog, $"{DateTime.Now:yyyy-MM-dd HH:mm:ss} ERROR: {ex}\n");
         }
-        catch { }
     }
 
     static void RunDashboard()