Change Password

Change user's master password by re-keying all workspace keys.

Client must:

1. Prove knowledge of current master password via X25519 public key
2. Provide new X25519 public key (derived from new master password)
3. Re-wrap all workspace keys with new X25519 public key

Server will:

1. Verify current master password (public key comparison)
2. Update X25519 public key
3. Update all workspace wrapped keys

Note: This changes the master password (encryption password), not authentication password. Both local and SSO users can change their master password.