### 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 "extensions/browser/extension_navigation_ui_data.h"
#include "extensions/browser/extension_prefs.h"
#include "extensions/browser/extension_protocols.h"
#include "extensions/browser/extension_registry.h"
#include "extensions/browser/extensions_browser_client.h"
#include "extensions/browser/guest_view/extensions_guest_view_message_filter.h"
#include "extensions/browser/guest_view/mime_handler_view/mime_handler_view_guest.h"
#include "extensions/browser/info_map.h"
#include "extensions/browser/process_manager.h"
#include "extensions/browser/process_map.h"
#include "extensions/browser/url_loader_factory_manager.h"
#include "extensions/common/api/mime_handler.mojom.h"
#include "extensions/common/constants.h"
#include "extensions/common/extension.h"
#include "extensions/common/switches.h"
#include "shell/browser/extensions/electron_extension_message_filter.h"
#include "shell/browser/extensions/electron_extension_system.h"
#include "shell/browser/extensions/electron_extension_web_contents_observer.h"
#endif
#if BUILDFLAG(ENABLE_PLUGINS)
#include "chrome/browser/plugins/plugin_response_interceptor_url_loader_throttle.h"  
#include "shell/browser/plugins/plugin_utils.h"
#endif
#if defined(OS_MAC)
#include "content/common/mac_helpers.h"
#include "content/public/common/child_process_host.h"
#endif
#if defined(OS_LINUX) && !defined(MAS_BUILD)
#include "base/debug/leak_annotations.h"
#include "components/crash/content/browser/crash_handler_host_linux.h"