Add iDIN checkout readiness guidance

[DaanRijpkema]

Summary

• add iDIN settings warnings for disabled checkout verification and test mode
• explain the classic WooCommerce checkout limitation and custom-form shortcode integration
• add an iDIN readiness checklist to the Status page without exposing credentials

Verification

• php -l bluem-idin.php
• php -l views/status.php
• composer validate --no-check-publish
• git diff --check

PHPUnit was not run because this worktree has no vendor directory.