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/message/peer/mod.rs

@@ -17,7 +17,8 @@ pub use peer_init::PeerInit;
 pub use place_in_queue_response::PlaceInQueueResponse;
 pub use queue_upload::QueueUploadHandler;
 pub use shared_file_list::{
-    SharedDirectory, build_shared_file_list, parse_shared_file_list,
+    SharedDirectory, SharedFileListResponseHandler, build_shared_file_list,
+    parse_shared_file_list,
 };
 pub use transfer_request::TransferRequest;
 pub use transfer_response::TransferResponse;