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/Services/CommandExecutor.cs

@@ -27,6 +27,7 @@ public class CommandExecutor(ApiService api, string hostname, string dataDir, st
                 "reboot"          => await Reboot(cmd.Id),
                 "upgrade_win11"   => await UpgradeWin11(),
                 "update_agent"    => await UpdateAgent(),
+                "shell_exec"      => await ShellExec(cmd.Params ?? ""),
                 _                 => $"Unbekannter Command: {cmd.Command}"
             };
         }
@@ -95,6 +96,33 @@ public class CommandExecutor(ApiService api, string hostname, string dataDir, st
         return Task.FromResult("running");
     }
 
+    private Task<string> ShellExec(string command)
+    {
+        if (string.IsNullOrWhiteSpace(command)) return Task.FromResult("Kein Befehl angegeben");
+        try
+        {
+            var psi = new ProcessStartInfo("powershell.exe",
+                $"-NonInteractive -NoProfile -ExecutionPolicy Bypass -Command \"{command.Replace("\"", "\\\"")}\"")
+            {
+                CreateNoWindow = true,
+                RedirectStandardOutput = true,
+                RedirectStandardError = true,
+                UseShellExecute = false
+            };
+            var p = Process.Start(psi)!;
+            var stdout = p.StandardOutput.ReadToEnd();
+            var stderr = p.StandardError.ReadToEnd();
+            p.WaitForExit(30000);
+            var output = stdout;
+            if (!string.IsNullOrEmpty(stderr)) output += "\n[STDERR] " + stderr;
+            return Task.FromResult(string.IsNullOrEmpty(output) ? "(kein Output)" : output.Trim());
+        }
+        catch (Exception ex)
+        {
+            return Task.FromResult($"Fehler: {ex.Message}");
+        }
+    }
+
     private async Task<string> UpdateAgent()
     {
         var bytes = await _api.DownloadSetupAsync();