fix(keys): a device refuses Argon2id costs beyond the register's ceiling

PasswordKdf.fromJson refused costs below the floor but took any above it,
so an instance could ask a device for 100 GiB and crash the app at every
sign-in. It now refuses more than the register's own ceiling (16 passes,
1 GiB; KeyRefusal.excessivePasswordKdf), and the app reads either refusal,
before deriving or sending anything, as PasswordProblem.unsafeCosts with
a text of its own (before, costs below the floor read as keys missing).

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/sodium_account_crypto_test.dart

@@ -43,6 +43,32 @@ void main() {
     signingPublicKey: account.keys.signingPublicKey,
   );
 
+  test("refuses costs an instance mustn't ask for, before deriving "
+      'anything: below the floor, or beyond what a device can spare', () async {
+    for (final costs in [
+      protocol.PasswordCosts(
+        algorithm: 'argon2id13',
+        opsLimit: 1,
+        memLimit: 8 * 1024 * 1024,
+      ),
+      protocol.PasswordCosts(
+        algorithm: 'argon2id13',
+        opsLimit: 3,
+        memLimit: 100 * 1024 * 1024 * 1024,
+      ),
+    ]) {
+      await expectLater(
+        crypto.unlock(
+          'correct horse battery staple',
+          salt: Uint8List.sublistView(account.setup.salt),
+          costs: costs,
+        ),
+        failsWith(PasswordProblem.unsafeCosts),
+        reason: '${costs.memLimit}',
+      );
+    }
+  });
+
   test('a new account: what the register takes, its keys opening with the '
       'password on another device, the verifier made of the same x', () async {
     final setup = account.setup;