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/cli.rs

@@ -43,9 +43,21 @@ pub struct Cli {
     #[command(subcommand)]
     pub command: Option<Commands>,
 
-    #[arg(short, long, default_value = "~/Downloads")]
+    #[arg(
+        short,
+        long,
+        env = "SOULSEEK_DOWNLOAD_DIR",
+        default_value = "~/Downloads"
+    )]
     pub download_dir: String,
 
+    #[arg(
+        long,
+        env = "SOULSEEK_SHARED_DIR",
+        help = "Directory whose files are shared with (uploaded to) other peers"
+    )]
+    pub shared_dir: Option<String>,
+
     #[arg(
         short = 'c',
         long,