[native] Use useRegistrationServerCall in FullscreenSIWEPanel for identity registration

Summary:
We had an isolated call to useIdentityWalletRegisterCall here, which doesn't handle keyserver auth. By replacing it with a call to useRegistrationServerCall (via RegistrationContext) we make sure that keyserverAuth happens after identity registration, since it's no longer owned by KeyserverConnectionHandler.

Depends on D11766

Test Plan:
This whole stack was tested with the following steps:

1. On native, attempting to log in with an ETH account that hasn’t been registered yet
2. On native, attemping to register with an ETH account that has already been registered
3. On native, log in with an ETH account that has already been registered
4. On native, register with an ETH account that hasn’t already been registered
5. On native, register with a password account
6. On native, log in with a password account
7. On web, attempting to log in with an ETH account that hasn’t been registered yet
8. On web, log in with an ETH account that has already been registered
9. On web, log in with a password account

Reviewers: tomek, inka

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D11767