### 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
frame->GetRoutingID(), frame->GetLastCommittedOrigin(), browser_context,
      &next_id_);
}
bool ElectronBrowserClient::WillCreateURLLoaderFactory(
    content::BrowserContext* browser_context,
    content::RenderFrameHost* frame_host,
    int render_process_id,
    URLLoaderFactoryType type,
    const url::Origin& request_initiator,
    absl::optional<int64_t> navigation_id,
    ukm::SourceIdObj ukm_source_id,
    mojo::PendingReceiver<network::mojom::URLLoaderFactory>* factory_receiver,
    mojo::PendingRemote<network::mojom::TrustedURLLoaderHeaderClient>*
        header_client,
    bool* bypass_redirect_checks,
    bool* disable_secure_dns,
    network::mojom::URLLoaderFactoryOverridePtr* factory_override) {
  v8::Isolate* isolate = JavascriptEnvironment::GetIsolate();
  v8::HandleScope scope(isolate);
  auto web_request = api::WebRequest::FromOrCreate(isolate, browser_context);
  DCHECK(web_request.get());
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  if (!web_request->HasListener()) {
    auto* web_request_api = extensions::BrowserContextKeyedAPIFactory<
        extensions::WebRequestAPI>::Get(browser_context);
    DCHECK(web_request_api);
    bool use_proxy_for_web_request =
        web_request_api->MaybeProxyURLLoaderFactory(
            browser_context, frame_host, render_process_id, type, navigation_id,
            ukm_source_id, factory_receiver, header_client);