chore: replace userdir with stdlib os.UserConfigDir

[yin1999]

The github.com/vrischmann/userdir package has been archived. We can drop this dependency and leverage the standard library's os package instead.

[tiny-craft]

Thanks for the PR. Have you tested this change on all supported platforms, especially macOS, Windows, Linux, and the web build?

One concern is backward compatibility with the existing config path. For example, on macOS the old userdir.GetConfigHome() path appears to be ~/Library/Preferences, while os.UserConfigDir() uses ~/Library/Application Support. This may cause existing users' connections and preferences to no longer be found after upgrading.

Could you confirm whether the old paths are still supported, or add a migration/fallback mechanism for existing config files?