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/src/config.rs

@@ -10,4 +10,5 @@ pub struct SearchConfig {
     pub download_dir: String,
     pub verbose: u8,
     pub max_concurrent_downloads: usize,
+    pub shared_directory: Option<String>,
 }