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/data_keys_and_content_test.dart

@@ -109,6 +109,24 @@ Future<void> main() async {
         throwsArgumentError,
       );
     });
+
+    test(
+      'a sealed key too short to hold a box is refused, never thrown on',
+      () {
+        // Signed by the sealer, so only the box's own length can fail it.
+        final signature = sodium.crypto.sign.detached(
+          message: Uint8List(0),
+          secretKey: account.signing.secretKey,
+        );
+        for (final box in [Uint8List(1), Uint8List(47)]) {
+          expect(
+            () => open(Uint8List.fromList([...signature, ...box])),
+            throwsA(isA<KeyRefused>()),
+            reason: '${box.length}',
+          );
+        }
+      },
+    );
   });
 
   group('content, sealed and signed', () {