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.Tests/Security/Authorization/GatewayGrpcAuthorizationInterceptorTests.cs

@@ -458,6 +458,15 @@ public sealed class GatewayGrpcAuthorizationInterceptorTests
             return Task.FromResult(new SessionCloseResult(sessionId, SessionState.Closed, AlreadyClosed: false));
         }
 
+        /// <inheritdoc />
+        public Task<SessionCloseResult> KillWorkerAsync(
+            string sessionId,
+            string reason,
+            CancellationToken cancellationToken)
+        {
+            return Task.FromResult(new SessionCloseResult(sessionId, SessionState.Closed, AlreadyClosed: false));
+        }
+
         /// <inheritdoc />
         public Task<int> CloseExpiredLeasesAsync(
             DateTimeOffset now,