feat(app): the keys of accounts that sign in with SimpleLogin, on the device (ADR 0020, D19)

AccountCrypto makes the keys of an account that signs in with SimpleLogin
(newKeys): new key pairs or an anonymous person's device keys, wrapped
under the password and a new recovery phrase, with the public half of the
password's proof key in place of SRP's verifier. It proves the password to
open them (proveKeysPassword: one Argon2id for the proof and the unwrapping)
and the recovery phrase to link a new SimpleLogin account in place of a
lost one (signMendProof, bound to the attempt). The interop tests check all
three against the register's own checks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_base_server/lib/base.dart

@@ -47,6 +47,7 @@ export 'src/password/password_config.dart';
 export 'src/password/passwords.dart'
     show
         gitHubSignInFreshFor,
+        isKeysSetup,
         isNewPassword,
         isPasswordSetup,
         newPasswordCosts,