### 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
#include "electron/grit/electron_resources.h"
#include "electron/shell/common/api/api.mojom.h"
#include "mojo/public/cpp/bindings/binder_map.h"
#include "net/base/escape.h"
#include "net/ssl/ssl_cert_request_info.h"
#include "ppapi/buildflags/buildflags.h"
#include "ppapi/host/ppapi_host.h"
#include "printing/buildflags/buildflags.h"
#include "services/device/public/cpp/geolocation/location_provider.h"
#include "services/network/public/cpp/features.h"
#include "services/network/public/cpp/resource_request_body.h"
#include "services/network/public/cpp/self_deleting_url_loader_factory.h"
#include "shell/app/electron_crash_reporter_client.h"
#include "shell/browser/api/electron_api_app.h"
#include "shell/browser/api/electron_api_crash_reporter.h"
#include "shell/browser/api/electron_api_protocol.h"
#include "shell/browser/api/electron_api_session.h"
#include "shell/browser/api/electron_api_web_contents.h"
#include "shell/browser/api/electron_api_web_request.h"
#include "shell/browser/badging/badge_manager.h"
#include "shell/browser/child_web_contents_tracker.h"
#include "shell/browser/electron_autofill_driver_factory.h"
#include "shell/browser/electron_browser_context.h"
#include "shell/browser/electron_browser_handler_impl.h"
#include "shell/browser/electron_browser_main_parts.h"
#include "shell/browser/electron_navigation_throttle.h"
#include "shell/browser/electron_quota_permission_context.h"
#include "shell/browser/electron_speech_recognition_manager_delegate.h"
#include "shell/browser/font_defaults.h"
#include "shell/browser/javascript_environment.h"