fix(dashboard)!: move login POST to /auth/login to resolve AmbiguousMatchException

The themed Blazor <LoginCard> page (Components/Pages/Login.razor, @page "/login")
registers a Razor Components endpoint that matches ALL HTTP methods. The credential
form POSTed to /login, where MapPost("/login") also matched — so every POST /login
threw Microsoft.AspNetCore.Routing.Matching.AmbiguousMatchException (HTTP 500),
breaking dashboard login for every user. It was latent because the dashboard was only
ever reached via the AllowAnonymousLocalhost bypass on the host box.

Move the credential POST to a distinct /auth/login route (mirroring ScadaBridge, which
never collided because it posts to /auth/login). GET /login stays the Blazor page; the
cookie LoginPath stays /login. Adds a registration assertion pinning DashboardLoginPost
to /auth/login as the regression guard.

Files: Login.razor (LoginCard Action), DashboardEndpointRouteBuilderExtensions (MapPost
route), GatewayApplicationTests (route assertion).

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

@@ -29,8 +29,14 @@ public static class DashboardEndpointRouteBuilderExtensions
         // kit's <LoginCard>. Its [AllowAnonymous] attribute overrides the
         // RequireAuthorization(ViewerPolicy) that MapRazorComponents<App>() applies,
         // so the cookie scheme's LoginPath="/login" redirect resolves for anonymous users.
+        //
+        // The credential POST is mapped to /auth/login, NOT /login. The @page "/login"
+        // Razor Components endpoint matches ALL HTTP methods, so a MapPost("/login") shared
+        // the "/login" route with it and every POST threw AmbiguousMatchException (HTTP 500).
+        // A distinct /auth/login path (as ScadaBridge does) keeps the GET page and the POST
+        // handler on separate routes. The <LoginCard Action="/auth/login"> form posts here.
         endpoints.MapPost(
-                "/login",
+                "/auth/login",
                 (HttpContext httpContext, IAntiforgery antiforgery, IDashboardAuthenticator authenticator) =>
                     PostLoginAsync(httpContext, antiforgery, authenticator))
             .AllowAnonymous()