message: Ask for passphrase at Mint dialog
diff: @@ -121,6 +121,10 @@ void SigmaPage::on_mintButton_clicked()
  amount = amount / CENT * CENT + ((amount % CENT >= CENT / 2) ? CENT : 0);
  try {
  + WalletModel::UnlockContext ctx(walletModel->requestUnlock());
  + if (!ctx.isValid()) {
  + return;
  + }
  walletModel->sigmaMint(amount);
  } catch (const std::runtime_error& e) {
  QMessageBox::critical(this, tr("Error"),
  