Dashboard: admin-only Close session / Kill worker

Add IDashboardSessionAdminService (Admin-role gate, friendly errors,
audit logging) wrapping a new ISessionManager.KillWorkerAsync that
skips graceful shutdown and cleans up registry/metrics. Sessions,
Workers, and SessionDetails pages render Close / Kill buttons only
when CanManage; the service re-checks the role on every call so
forged clicks return Unauthenticated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardSessionAdminResult.cs

@@ -0,0 +1,16 @@
+namespace ZB.MOM.WW.MxGateway.Server.Dashboard;
+
+public sealed record DashboardSessionAdminResult(
+    bool Succeeded,
+    string Message)
+{
+    public static DashboardSessionAdminResult Success(string message)
+    {
+        return new DashboardSessionAdminResult(true, message);
+    }
+
+    public static DashboardSessionAdminResult Fail(string message)
+    {
+        return new DashboardSessionAdminResult(false, message);
+    }
+}