### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

main

### What operating system are you using?

All OSes.

### What arch are you using?

All archs.

### Last Known Working Electron version

N/A

### Expected Behavior

[`ElectronBrowserClient::GetDefaultDownloadDirectory()`](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019) is hardcoded to return `~/Downloads`. However, this might not be the actual downloads directory path (eg: if the user has changed it or on OSes which translate user directory names).

The proper way to get the current user's downloads directory is OS-dependent:
- On Linux it is set in the env var `$XDG_DOWNLOAD_DIR` 
- On Windows it is returned by [`SHGetKnownFolderPath`](https://docs.microsoft.com/en-us/windows/win32/api/shlobj_core/nf-shlobj_core-shgetknownfolderpath) passing in `FOLDERID_Downloads`
- On MacOS it is returned by [`URLForDirectory`](https://developer.apple.com/documentation/foundation/nsfilemanager/1407693-urlfordirectory) passing in `NSDownloadsDirectory`

### Current Behavior

`~/Downloads` is [hardcoded](https://github.com/electron/electron/blob/main/shell/browser/electron_browser_client.cc#L1013-L1019).

shell/browser/electron_browser_client.cc
void CreateLoaderAndStart(
      mojo::PendingReceiver<network::mojom::URLLoader> loader,
      int32_t request_id,
      uint32_t options,
      const network::ResourceRequest& request,
      mojo::PendingRemote<network::mojom::URLLoaderClient> client,
      const net::MutableNetworkTrafficAnnotationTag& traffic_annotation)
      override {
    if (!content::ChildProcessSecurityPolicy::GetInstance()->CanRequestURL(
            child_id_, request.url)) {
      mojo::Remote<network::mojom::URLLoaderClient>(std::move(client))
          ->OnComplete(
              network::URLLoaderCompletionStatus(net::ERR_ACCESS_DENIED));
      return;
    }
    content::CreateFileURLLoaderBypassingSecurityChecks(
        request, std::move(loader), std::move(client),
        nullptr,
         true);
  }
  int child_id_;
  DISALLOW_COPY_AND_ASSIGN(FileURLLoaderFactory);
};
}  
#endif  
void ElectronBrowserClient::RegisterNonNetworkSubresourceURLLoaderFactories(