fix(app): no sheet is left busy by an error nothing expected

An instance's salt of another length made the device's cryptography throw
an ArgumentError, a wrapping's nonce a RangeError, and libsodium or the
keyring may throw their own; none was a refusal, so the step's error
escaped and the sheet stayed busy, which can't be closed. halkyon_keys
now checks the lengths of what comes from elsewhere before libsodium sees
it (KeyRefusal.malformed: a salt, a wrapping, a sealed key's box, a sealed
payload), the device's cryptography turns anything else into the general
failure, and so does every password cubit, the error logged by its type
alone.

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_keys/test/password_keys_test.dart

@@ -54,17 +54,29 @@ Future<void> main() async {
     expect(keys.srpPrivateValue, greaterThan(BigInt.zero));
   });
 
-  test('a salt of another length is refused', () {
-    expect(
-      () => derivePasswordKeys(
-        sodium,
-        password: 'a password',
-        salt: Uint8List(15),
-        kdf: cheap,
-      ),
-      throwsArgumentError,
-    );
-  });
+  test(
+    'a salt of another length, as an instance might send one, is refused',
+    () {
+      for (final length in [0, 15, 17]) {
+        expect(
+          () => derivePasswordKeys(
+            sodium,
+            password: 'a password',
+            salt: Uint8List(length),
+            kdf: cheap,
+          ),
+          throwsA(
+            isA<KeyRefused>().having(
+              (refused) => refused.reason,
+              'reason',
+              KeyRefusal.malformed,
+            ),
+          ),
+          reason: '$length',
+        );
+      }
+    },
+  );
 
   test('new salts are 16 random bytes', () {
     final first = newPasswordSalt(sodium);