feat: configure download & upload (shared) directories

Add a --shared-dir / SOULSEEK_SHARED_DIR option and a SOULSEEK_DOWNLOAD_DIR
env for the download directory. The shared directory is resolved and
validated up front (tilde-expanded, must exist and be a directory); a
misconfigured value is a warning, not a fatal error.

The library gains an optional shared_directory on ClientSettings/Client
with a getter, keeping soulseek-rs-lib dependency-free. Resolution lives
in a new soulseek-rs/src/directories.rs with unit tests.

soulseek-rs-lib/tests/e2e.rs

@@ -91,6 +91,7 @@ impl TestServer {
             server_address: PeerAddress::new(self.host.clone(), self.port),
             enable_listen: false,
             listen_port: 0,
+            shared_directory: None,
         }
     }
 }