Fix locale-dependent decimal parsing in CSV reader

[Pana1v]

Replace std::stod() with locale-independent std::from_chars() to ensure decimal numbers are parsed correctly regardless of system locale settings.

Fixes #1237

[facontidavide]

doesn't compile for macos!

[facontidavide]

I found a different solution using Qt provided function, that I think is better for portability.

FYI:

std::optional<double> toDouble(const std::string& str)
{
  // If comma is used as decimal separator (European format), replace with dot
  QString qstr = QString::fromStdString(str);
  qstr.replace(',', '.');

  // Use QLocale::c() for locale-independent parsing (always uses '.' as decimal separator)
  bool ok = false;
  double value = QLocale::c().toDouble(qstr, &ok);
  return ok ? std::optional<double>(value) : std::nullopt;
}

thanks a lot anyway for looking into this