feat: browse another user's shared files

Add Client::browse_user (sends GetShareFileList, code 4, connecting first if
needed via the same queue-and-flush path as search replies) and
take_browse_result. A new SharedFileListResponseHandler parses the peer's
code-5 reply into SharedDirectory entries, forwarded as
ClientOperation::BrowseResult and stored per user. Verified end to end: a
second real client browses a sharer and receives its file listing.

soulseek-rs-lib/src/lib.rs

@@ -24,4 +24,5 @@ pub mod prelude {
 pub use actor::server_actor::{PeerAddress, UserMessage};
 pub use client::{Client, ClientSettings};
 pub use error::{Result, SoulseekRs};
+pub use message::peer::SharedDirectory;
 pub use types::{DownloadStatus, File, Search, SearchResult, Transfer};