feat: implement §9 file transfer over dedicated binary DataChannels

File chunks travel over a per-transfer "f:<xid>" WebRTC DataChannel —
direct DTLS-encrypted P2P, the anchor never sees file data. Once the
initial handshake is done the anchor can disappear and transfers continue.

Key design choices:
- Receiver sends "ok" on the file DC before sender streams, eliminating
  a race where tiny files could be fully sent/closed before the receiver's
  OnMessage handler is registered (open-race §6 analogue for data DCs).
- Auto-accept: receiver accepts every incoming offer immediately.
- Download dir: per-network at <data-dir>/downloads-<netid>/.
- Backpressure: bufferedAmountLowThreshold to avoid overwhelming the DC.
- SHA-256 verified on receive; mismatches emit EvtError and discard temp file.
- IPC: send_file {peer_id, path} → offers the named file from share dir.
- EvtFileComplete {transfer_id, path} emitted on success.

IPC command: {"type":"send_file","peer_id":"<hex>","path":"<filename>"}

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

internal/mesh/peer.go

@@ -57,6 +57,7 @@ func WireDataChannel(
 		peerConn := &PeerConn{
 			Info: proto.PeerInfo{ID: peerID, Alias: string(peerID.Short())},
 			Send: sendCh,
+			PC:   pc,
 		}
 		m.AddPeer(peerConn)
 
@@ -211,6 +212,16 @@ func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 			PeerID: peerIDPtr(from),
 			Offer:  &proto.FileOffer{Xid: msg.Xid, Name: msg.Name, Size: msg.Size, SHA256: msg.SHA256},
 		})
+		m.acceptIncoming(msg, from)
+
+	case proto.MsgFileAccept:
+		m.startSend(msg.Xid, from)
+
+	case proto.MsgFileCancel:
+		log.Printf("mesh: file-cancel from %s xid=%s reason=%s", from.Short(), msg.Xid, msg.Reason)
+
+	case proto.MsgFileDone:
+		log.Printf("mesh: file-done from %s xid=%s", from.Short(), msg.Xid)
 
 	case proto.MsgPeerGossip:
 		if msg.Gossip != nil {