feat(app): content sealed with data keys, and an anonymous person's device keys (ADR 0019)

`ContentCipher` makes a person's data keys, sealed to their account and
signed by it, opens them, and seals and opens a row's content with them for
the row it is stored in (`SodiumContentCipher`, on the isolate it is used
on, as rows are read and written one by one; `FakeContentCipher` for
tests). A sealed payload holds the signature, then the ciphertext.

`AccountCrypto.newDeviceKeys` makes the key pairs an anonymous person's
device encrypts their private studysets with (ADR 0017, point 9, not built
before); setting a password keeps them as the account's
(`keepingKeys` with a new phrase), so nothing they made private is lost.

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_app/lib/testing.dart

@@ -5,6 +5,7 @@ library;
 
 export 'src/testing/fake_account_crypto.dart';
 export 'src/testing/fake_auth_gateway.dart';
+export 'src/testing/fake_content_cipher.dart';
 export 'src/testing/fake_instance_checker.dart';
 export 'src/testing/fake_password_gateway.dart';
 export 'src/testing/fake_screen_guard.dart';