Fix file transfer, signaling reliability, and DC callback races

- wireFileRecv: replace OnMessage/OnClose channel approach with pion
  Detach API so SCTP buffers data regardless of callback timing — fixes
  0-byte files caused by pion dispatching data goroutines before OnMessage
  was registered
- yaw DC: same Detach API treatment for the control channel
- connectTo: use DetachDataChannels SettingEngine, replace stale PCs
  (Connecting/Failed/Disconnected state) instead of returning early;
  isCurrent guard in OnConnectionStateChange prevents stale-PC close
  from firing OnDisconnected
- offerByOrder: gate maybeOffer on peer ID order so only one side
  creates the yaw DC and SDP offer — fixes signaling glare causing
  triple connected (verified) logs
- pendingRecvs: promote single pendingRecv slot to map[string]*recvState
  so concurrent file offers don't overwrite each other
- peer-left: stop calling resetPeer() so active file DCs aren't closed
  before data arrives
- signaling ping: wrap conn.Ping with 10s timeout context so a dead TCP
  connection is detected within 30s rather than hanging forever
- signaling read: 3-minute read deadline forces reconnect if anchor
  silently evicts the peer from the room (idle timeout)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,6 +3,7 @@ cli/flit
 
 # Host-specific deploy scripts (contain server addresses / SSH targets)
 deploy-pwa.sh
+deploy-daemon.sh
 serve-pwa.sh
 
 # Runtime config — contains anchor URL; copy from example and fill in