Fix UI sync bugs and disconnect hang

- Add NeighborDown event handler in gossip network to reliably remove disconnected peers.
- Add 1-second timeout wrapper to router.shutdown() to prevent UI hanging on disconnect.

src/network/gossip.rs

@@ -195,6 +195,11 @@ impl RoomState for IrohGossipState {
                     }
                     Ok(iroh_gossip::api::Event::NeighborDown(peer_id)) => {
                         crate::log_msg(&format!("Gossip event: NeighborDown={:?}", peer_id));
+                        let removed = peers.lock().unwrap().remove(&peer_id).is_some();
+                        if removed {
+                            crate::log_msg(&format!("Removed peer due to NeighborDown: {:?}", peer_id));
+                            let _ = event_tx.send(RoomEvent::PeerLeft(peer_id)).await;
+                        }
                     }
                     Ok(other) => {
                         crate::log_msg(&format!("Gossip other event: {:?}", other));