fix(peer): reset cancelled outbound file streams

Cancelled outbound transfers previously returned from the streaming loop without
terminating the QUIC send half. A whole-file receiver relies on the stream
ending to distinguish EOF from an in-progress body, so cancellation could leave
it waiting on a truncated transfer until its own timeout fired.

Reset the send stream on every cancellation branch, including cancellation
while waiting for the final close acknowledgement. A reset is deliberately used
instead of a graceful close so truncated whole-file transfers cannot be
misinterpreted as a valid EOF.

Test Plan:
- just test
- just clippy
- git diff --check

Refs: Claude review finding #1

crates/lanspread-peer/src/peer.rs

@@ -4,6 +4,7 @@ use bytes::Bytes;
 use lanspread_db::db::GameFileDescription;
 use lanspread_utils::maybe_addr;
 use s2n_quic::{
+    application,
     connection,
     stream::{Error as StreamError, SendStream},
 };
@@ -14,6 +15,16 @@ use tokio::{
 
 use crate::{config::FILE_TRANSFER_BUFFER_SIZE, path_validation::validate_game_file_path};
 
+fn cancel_send_stream(tx: &mut SendStream, remote_addr: impl std::fmt::Display, path: &Path) {
+    // Reset instead of finishing so truncated whole-file transfers cannot look like EOF.
+    if let Err(err) = tx.reset(application::Error::UNKNOWN) {
+        log::debug!(
+            "{remote_addr} failed to reset cancelled transfer for {}: {err}",
+            path.display()
+        );
+    }
+}
+
 #[allow(clippy::too_many_lines)]
 async fn stream_file_bytes(
     tx: &mut SendStream,
@@ -52,6 +63,7 @@ async fn stream_file_bytes(
                 "{remote_addr} transfer cancelled for {}",
                 validated_path.display()
             );
+            cancel_send_stream(tx, remote_addr, &validated_path);
             return Err(eyre::eyre!("File transfer cancelled by user"));
         }
 
@@ -67,6 +79,7 @@ async fn stream_file_bytes(
                     "{remote_addr} transfer cancelled for {}",
                     validated_path.display()
                 );
+                cancel_send_stream(tx, remote_addr, &validated_path);
                 return Err(eyre::eyre!("File transfer cancelled by user"));
             }
             res = file.read(&mut buf[..read_len]) => {
@@ -86,6 +99,7 @@ async fn stream_file_bytes(
                     "{remote_addr} transfer cancelled for {}",
                     validated_path.display()
                 );
+                cancel_send_stream(tx, remote_addr, &validated_path);
                 return Err(eyre::eyre!("File transfer cancelled by user"));
             }
             res = tx.send(Bytes::copy_from_slice(&buf[..bytes_read])) => {
@@ -132,6 +146,7 @@ async fn stream_file_bytes(
     tokio::select! {
         () = cancel_token.cancelled() => {
             log::info!("{remote_addr} transfer cancelled while closing stream");
+            cancel_send_stream(tx, remote_addr, &validated_path);
             return Err(eyre::eyre!("File transfer cancelled by user"));
         }
         res = tx.close() => {