Function spawn_receive_handler 

pub fn spawn_receive_handler<N: NetworkProvider + 'static>(
    node: Arc<Node<N>>,
    offer_tx: UnboundedSender<(FileOffer, OfferResponder)>,
    event_tx: Sender<FileTransferEvent>,
) -> JoinHandle<()>

Spawn a background task that listens for incoming file transfer messages.

• OFFER: Creates a FileOffer + OfferResponder pair, sends them on the offer_tx channel, and waits up to 60 seconds for a decision.
• PULL_REQUEST: Auto-serves the requested file (same as CLI).
• ACCEPT / REJECT: Ignored (handled by send/pull initiators).