[AnVIL Explorer] Collapsible registration completion reminder

[NoopDog]

Summary

Make the registration completion reminder (onboarding wizard) on AnVIL Explorer collapsible so users can dismiss it while browsing without having to fully complete registration.

Scope

AnVIL Explorer only.

Behavior

• Add a way to collapse/hide the registration completion reminder.
• Remember the user's choice to hide the reminder for the duration of the session.
  • Session scope only — do not use localStorage.
  • The hidden state must persist across page navigations within the session.
  • When the session ends, the reminder reappears on next visit.

Design

• Design ticket: clevercanary/cc-design#308
• Figma link is in the design ticket.