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/test/password/data/account_key_store_test.dart

@@ -0,0 +1,19 @@
+import 'package:flutter_secure_storage/flutter_secure_storage.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+
+void main() {
+  test("the account's keys stay on this device on Apple's systems: never in "
+      'a backup restored elsewhere, nor in iCloud Keychain', () {
+    for (final options in [
+      accountKeyStorage.iOptions,
+      accountKeyStorage.mOptions,
+    ]) {
+      expect(
+        options.accessibility,
+        KeychainAccessibility.first_unlock_this_device,
+      );
+      expect(options.synchronizable, isFalse);
+    }
+  });
+}