### 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
return "Chrome/" CHROME_VERSION_STRING;
}
std::string ElectronBrowserClient::GetUserAgent() {
  if (user_agent_override_.empty())
    return GetApplicationUserAgent();
  return user_agent_override_;
}
void ElectronBrowserClient::SetUserAgent(const std::string& user_agent) {
  user_agent_override_ = user_agent;
}
void ElectronBrowserClient::RegisterNonNetworkNavigationURLLoaderFactories(
    int frame_tree_node_id,
    ukm::SourceIdObj ukm_source_id,
    NonNetworkURLLoaderFactoryMap* factories) {
  content::WebContents* web_contents =
      content::WebContents::FromFrameTreeNodeId(frame_tree_node_id);
  content::BrowserContext* context = web_contents->GetBrowserContext();
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  factories->emplace(
      extensions::kExtensionScheme,
      extensions::CreateExtensionNavigationURLLoaderFactory(
          context, ukm_source_id,
          false ));
#endif
  
  auto* protocol_registry = ProtocolRegistry::FromBrowserContext(context);
  protocol_registry->RegisterURLLoaderFactories(factories,
                                                true );
}
void ElectronBrowserClient::