Fix: invalidver opt-in auto-open

[Wimberton]

When using invalidver handling in my loaders, the library always opened the download URL and exited before my code could run. That prevented showing a custom error (e.g. console message + countdown) or using app_data.downloadLink in my own UI. This change makes the old auto-open-and-exit behavior opt-in via auto_open_download_url, and ensures response.message is set so callers can detect and display the error. Example of the flow I wanted to support:

if (KeyAuthApp.response.message == "invalidver") {
    std::cout << "Your loader is outdated!" << std::endl;
    if (!KeyAuthApp.app_data.downloadLink.empty()) {
        std::cout << "Please visit our website to download the latest version:" << std::endl;
        std::cout << KeyAuthApp.app_data.downloadLink << std::endl;
    } else {
        std::cout << "Please download the latest version." << std::endl; // e.g. when white labeling loaders
    }
    // e.g. countdown then return 1
}

The default is auto_open_download_url= true, so existing code keeps the original behavior (open URL and exit). No changes required for current users. I consider that behavior broken for loaders that want to handle the error themselves; set auto_open_download_url = false before init() to get the new behavior.

Summary

• Opt-in caller handling: A new auto_open_download_url flag (default true) controls invalidver behavior. When true, behavior is unchanged: open download URL if set, then exit. When false, the library stores the download link and returns so the caller can handle the error.
• Descriptive error: When the server returns invalidver, response.message is now set so callers can show a clear message:
  • If the dashboard download link is blank: a full diagnostic string (version mismatch + how to fix).
  • If the download link is present: "invalidver" so the caller can detect and display it.

Behavior

auto_open_download_url	Result
true (default)	Same as before: open URL if non-empty, then exit(0). No change for existing users.
false	No URL open, no exit; app_data.downloadLink and response.message are set for the caller.

Files

• auth.hpp: added bool auto_open_download_url = true;
• auth.cpp: invalidver block updated to set response.message and to open/exit only when auto_open_download_url is true

[Wimberton]

Silly me.
I've updated the variable to match the project snake-case.