Dashboard: handle GET /logout (was 405) by signing out + redirecting to /login

Browsers that navigate directly to /logout via the address bar issued a GET
against a POST-only route and got 405 Method Not Allowed. Logout is
self-destructive, so the GET path can skip antiforgery; the existing POST
form (used by the layout's Sign out button) is unchanged and still
antiforgery-protected.

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

src/ZB.MOM.WW.MxGateway.Tests/Gateway/GatewayApplicationTests.cs

@@ -63,7 +63,7 @@ public sealed class GatewayApplicationTests
         IReadOnlyList<RouteEndpoint> endpoints = GetRouteEndpoints(app);
 
         string[] anonymousEndpointNames =
-            ["DashboardLogin", "DashboardLoginPost", "DashboardLogout", "DashboardAccessDenied"];
+            ["DashboardLogin", "DashboardLoginPost", "DashboardLogout", "DashboardLogoutGet", "DashboardAccessDenied"];
         foreach (string endpointName in anonymousEndpointNames)
         {
             RouteEndpoint endpoint = Assert.Single(