Clarify diagnosis entry — sync onboarding diagnosis field with treatment plan

When adding a new client to Upheal, there's a diagnosis field in the client onboarding form. However, diagnoses are also recorded in the treatment plan's case snapshot. It's unclear which field is the source of truth, whether they sync, and why both exist. This creates confusion about where to enter diagnoses and whether updates in one place affect the other.

It would be helpful to either:

• Clarify the intended use of each field (onboarding diagnosis vs. treatment plan diagnoses)

• Sync them so a diagnosis entered during onboarding populates the treatment plan, or vice versa

• Remove the duplicate field and streamline diagnosis entry to a single location

This would make the workflow clearer for new users setting up clients.