fix(app): on Apple's systems the account's keys stay on this device

flutter_secure_storage's default accessibility on iOS and macOS
(unlocked) lets keychain items into encrypted backups, which restore on
another device. The account's unlocked keys are now kept
first_unlock_this_device (macOS through its data protection keychain, the
package's default there), and never synchronised to iCloud Keychain.

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/src/password/data/account_key_store.dart

@@ -4,6 +4,20 @@ import 'dart:developer';
 import 'package:flutter_secure_storage/flutter_secure_storage.dart';
 import 'package:halkyon_app/src/password/model/unlocked_account_keys.dart';
 
+/// The system's keyring as the account's keys are kept in it. On iOS and
+/// macOS they open once the device was first unlocked after starting and
+/// stay on this device alone (`first_unlock_this_device`): the default
+/// (`unlocked`) lets them into encrypted backups, restored on another
+/// device; and never into iCloud Keychain (not `synchronizable`).
+const accountKeyStorage = FlutterSecureStorage(
+  iOptions: IOSOptions(
+    accessibility: KeychainAccessibility.first_unlock_this_device,
+  ),
+  mOptions: MacOsOptions(
+    accessibility: KeychainAccessibility.first_unlock_this_device,
+  ),
+);
+
 /// Where the device keeps the account's private keys once unlocked (ADR
 /// 0017): in the system's keyring, as sessions are, one person at a time
 /// per instance, gone once they sign out or another person signs in. Tests
@@ -25,7 +39,7 @@ abstract interface class AccountKeyStore() {
 /// ID, the instance's own prefix (`InstanceStorage.keysOf`).
 class SecureAccountKeyStore(
   final String _prefix, {
-  final FlutterSecureStorage _storage = const FlutterSecureStorage(),
+  final FlutterSecureStorage _storage = accountKeyStorage,
 }) implements AccountKeyStore {
   String _keyOf(String person) => '$_prefix:$person';