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/test/password/data/password_repository_test.dart

@@ -325,6 +325,38 @@ void main() {
       expect(addresses.addresses['anonymous-1'], 'dana@example.org');
     });
 
+    test("an anonymous person's device keys become their account's as they "
+        'set a password, so their private studysets stay readable', () async {
+      await auth.signInAnonymously();
+      final device = await crypto.newDeviceKeys();
+      await keys.keep('anonymous-1', device);
+      final request = await passwords.sendCode(
+        'dana@example.org',
+        language: 'en',
+        purpose: MailCodePurpose.signUp,
+      );
+
+      final material = await passwords.makePassword('her password');
+      await passwords.setPassword(
+        material,
+        address: 'dana@example.org',
+        request: request,
+        code: gateway.codeFor('dana@example.org'),
+      );
+
+      expect(material.keys, device);
+      expect(keys.kept['anonymous-1'], device);
+    });
+
+    test('someone signed up anew gets new keys, never the keys kept on the '
+        'device for another', () async {
+      await keys.keep('anonymous-1', crypto.keysNumbered(1));
+
+      final material = await passwords.makePassword('a new password');
+
+      expect(material.keys, isNot(crypto.keysNumbered(1)));
+    });
+
     test("keeps only the signed-in person's keys: another signing in, or "
         'signing out, takes the others away', () async {
       await auth.signInAnonymously();