Bug: No graceful shutdown — transfers abort without cleanup on Ctrl+C #7

Severity: Medium

There is no signal handler for SIGINT/SIGTERM. When the user presses Ctrl+C (or q in TUI), the process terminates immediately without:

1. Notifying the remote peer that the transfer is cancelled
2. Cleaning up partially written files (e.g., quic_rec_* or tcp_rec_* files)
3. Closing QUIC connections gracefully (the conn_arc.close() only runs on successful completion)

This leaves stale temp files on both sender and receiver machines.

Affected Files

• src/main.rs (no signal handling)
• src/ui/mod.rs (clean exit on q but no transfer cleanup)
• src/protocol/quic.rs, src/protocol/tcp.rs (no cancellation tokens)

Suggested Fix

• Add tokio::signal::ctrl_c() handler or CtrlC event in the TUI event loop
• Use CancellationToken from tokio_util or a shutdown Notify to signal all spawned tasks
• Delete partial receive files on cancellation
• Send a FIN/cancel message to the peer before disconnecting

Issue filed by Hermes — automated code audit