Function validPin

common/transferprotocol.cpp:250–253

    }

    bool validPin(const std::string& pin)
    {
        return pin.size() == 4 && std::all_of(pin.begin(), pin.end(), [](unsigned char c) { return std::isdigit(c) != 0; });
    }

    std::optional<uint64_t> zipStreamSize(const std::vector<SendFile>& files, const std::vector<std::string>& dirs)
    {