fix(admin): resolve Medium code-review finding (Admin-006)

Emit <AntiforgeryToken /> in the MainLayout sign-out form and remove
.DisableAntiforgery() from the /auth/logout endpoint so UseAntiforgery()
validates the token. A tokenless POST now returns 400, preventing CSRF-logout.
Regression-guarded by AuthEndpointsTests.Logout_without_antiforgery_token_is_rejected.

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

src/Server/ZB.MOM.WW.OtOpcUa.Admin/Components/Layout/MainLayout.razor

@@ -45,6 +45,7 @@
                             .Where(c => c.Type.EndsWith("/role")).Select(c => c.Value))
                     </div>
                     <form method="post" action="/auth/logout">
+                        <AntiforgeryToken />
                         <button class="rail-btn" type="submit">Sign out</button>
                     </form>
                 </Authorized>