[Organizer Position Invite] Design pass on contract signing screen/copy

I think this page needs a design pass.

- X margins are a bit weird
- The "You" is incorrectly capitalized
- I think we should provide more context around what contract people are signing and why it's needed.

Start from the Organizer Position Invite contract signing screen shown in the issue and inspect its current layout and copy. Review the X margins, capitalization of “You,” and the explanation of what contract users are signing and why; done means these concerns are addressed consistently on the screen.

Written by the indexing model from the issue text.