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.
Port numbers are 16-bit; the codebase had listen_port/listener_port
typed as u32, inconsistent with PeerAddress.port (already u16).
Migrate the type all the way through the CLI, config, ClientSettings,
Client, ServerActor, and the listener entry point. The wire format is
preserved: build_set_wait_port_message now takes u16 and widens to u32
via .into() before write_int32. Also rename DEFALT_LISTEN_PORT to
DEFAULT_LISTEN_PORT.
michel