Show registration unavailable page when signup is disabled

Instead of showing the signup form and returning an internal error on
submit, the SignUpPage now queries signUpEnabled upfront and displays a
friendly message explaining that registration is not available, with a
link back to login.

Adds a signUpEnabled GraphQL query field on the connect/v1 API and
handles ErrSignupDisabled as a FORBIDDEN error in the SignUp resolver.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/iam/service.go

@@ -188,6 +188,10 @@ func NewService(
 	return svc, nil
 }
 
+func (s *Service) IsSignUpEnabled() bool {
+	return !s.disableSignup
+}
+
 func (s *Service) Run(ctx context.Context) error {
 	wg := sync.WaitGroup{}
 	ctx, cancel := context.WithCancelCause(ctx)