The default download directory is now the platform Downloads folder plus
a Soulseek subfolder (via the directories crate — XDG on Linux, known
folder on Windows) instead of a tilde path that only worked with a HOME
variable. Sharing follows the Soulseek convention of sharing what you
download: shared_dir defaults to the download folder, and an explicitly
empty shared_dir opts out. The folder is created at startup so first-run
share validation succeeds.