Harden secure login cookie

[ottenhoff]

Summary

• harden the long-lived secure login cookie with explicit Secure, HttpOnly, and SameSite=None attributes
• stop accepting legacy AesCtr ciphertext in the secure-cookie path and use AesOpenSSL directly
• add targeted secure-cookie tests for the cookie options and legacy-ciphertext rejection

Notes

• this intentionally limits the crypto cleanup to the secure-cookie flow
• AesCtr is still used elsewhere in the codebase, including badges, RPC, and other legacy-secret paths, so it was not deleted in this PR
• existing remember-me cookies created with the old secure-cookie format will no longer be accepted and will require a fresh login

Validation

• php -l lib/src/Crypt/SecureCookie.php
• php -l lib/tests/Crypt/SecureCookieTest.php
• custom PHP smoke test covering create/extract, cookie options, and legacy secure-cookie rejection

[csev]

@ottenhoff I have been thinking about removing the secure login cookie completely. It is already off by default. That was my first step towards complete removal. None of my production servers use it. Another Tsugi user went through a security audit and they did not like it much. That pushed me down the path of complete removal. Your PR can be merged and is harmless methinks - we can remove later.