Without Remember-me the session cookie was still being written via
`cookies.signed.permanent`, so it survived browser restart on shared
devices — surprising for a user who explicitly opted out of Remember-me.
Issue a browser-session cookie (no Expires) when remember_me is off;
the server-side Session#expires_at still bounds the 24h / 30d window.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
