feat: track uploads with progress and cancellation

serve_file streams in chunks, updating a shared byte counter and
aborting with Interrupted when a cancel flag is set. Each served upload
is registered (peer, virtual path, size, live progress, status) and
exposed via Client::uploads(); Client::cancel_upload flags a matching
in-progress transfer. e2e: the download test now also asserts the
uploader recorded a completed upload with the right peer and byte
count.

soulseek-rs-lib/src/types.rs

@@ -251,3 +251,24 @@ mod tests {
         assert_eq!(transfer.size, 0);
     }
 }
+
+/// Lifecycle of a file we are serving to a peer.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum UploadStatus {
+    InProgress,
+    Completed,
+    Cancelled,
+    Failed(String),
+}
+
+/// A snapshot of one upload for display purposes.
+#[derive(Debug, Clone)]
+pub struct UploadInfo {
+    /// The peer receiving the file.
+    pub username: String,
+    /// The peer-facing virtual path being served.
+    pub filename: String,
+    pub size: u64,
+    pub bytes_sent: u64,
+    pub status: UploadStatus,
+}