feat: conventional default download and share folders

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.

soulseek-rs/src/cli.rs

@@ -40,7 +40,8 @@ pub struct Cli {
     #[command(subcommand)]
     pub command: Option<Commands>,
 
-    /// Directory downloads are saved to (default: ~/Downloads)
+    /// Directory downloads are saved to (default: your Downloads/Soulseek
+    /// folder, which is also shared unless shared_dir says otherwise)
     #[arg(short, long, env = "SOULSEEK_DOWNLOAD_DIR")]
     pub download_dir: Option<String>,