feat(protocol): add file download support for avatars and icons

- Add Request::DownloadFile variant with oneshot reply
- Add ProtocolClient::download_avatar(client_uid) and download_icon(icon_id)
- Track pending file downloads by FiletransferHandle
- Handle StreamItem::FileDownload: read bytes from TCP stream
- Handle StreamItem::FiletransferFailed: map to ProtocolError
- Add ProtocolError::FileTransfer(String) variant
- Add path helper tests for avatar/icon download paths
- No tsclientlib types leak across the adapter boundary

crates/chanora_protocol/src/lib.rs

@@ -115,4 +115,8 @@ pub enum ProtocolError {
     /// should never see this; if they do, it is a mapping bug here.
     #[error("protocol backend: {0}")]
     Backend(String),
+
+    /// A file transfer failed while downloading protocol-owned assets.
+    #[error("file transfer failed: {0}")]
+    FileTransfer(String),
 }