fix: reset signUp identifiers when disabling registration

When registration is disabled, signUp.identifiers must be reset to
["username"] with verify:false. Otherwise Logto enforces email as a
mandatory profile field on all users, blocking username-only users
(like the admin) from signing in.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/vendor/EmailConnectorService.java

@@ -131,6 +131,11 @@ public class EmailConnectorService {
         } else {
             logtoClient.updateSignInExperience(Map.of(
                 "signInMode", "SignIn",
+                "signUp", Map.of(
+                    "identifiers", List.of("username"),
+                    "password", true,
+                    "verify", false
+                ),
                 "signIn", Map.of(
                     "methods", List.of(
                         Map.of("identifier", "username", "password", true, "verificationCode", false, "isPasswordPrimary", true)