feat: show modal for user agreement

- Explain the why we need this information and how we intent to use it

- Make clear what we will be public and what is private

Do this so that a member can accept the service agreement and control which pieces of information are made public.

The issue names no files, tests, or entry points; begin by locating the member-facing flow where service agreements are accepted. Done means a modal explains why information is needed, distinguishes public from private data, lets a member accept the agreement, and controls which information is public.

Written by the indexing model from the issue text.