[AGE-1840] Update First-Time Admin User Signup Experience in OSS

[mmabrouk]

Current Situation: When a user first deploys Agenta open source, the first user to sign up becomes an admin user with exclusive rights to invite others to the platform. However, the current signup screen does not indicate this special admin status or the responsibilities that come with it.

Objective: Update the signup screen design to clearly inform users that they are registering as the first admin user with exclusive invitation privileges.

Requirements:

1. Clearly communicate admin status during initial signup
2. Explain the significance of being the first admin (invitation privileges)
3. Maintain a clean, user-friendly interface
4. Remove the current Terms of Service reference as discussed

_{From SyncLinear.com | AGE-1840}

[mmabrouk]