Add peer-to-peer file drop

Files move directly between peers over the overlay, with no server in the
path. The receiver listens on the WireGuard address only, so the port is
unreachable from outside the tunnel, and every offer is matched to a known
peer before anything is read.

Consent is the default: an offer carries metadata alone, and no payload
moves until the receiver accepts. Policy is per profile and device-local —
off, ask, or auto-accept, with per-sender exceptions on top.

Policy and history live in the profile's preferences, so removing a profile
takes its file drop state with it. Transfers interrupted by a restart are
settled on load; nothing survives to finish them, and left alone they would
sit in the log as permanently pending.

The Android bindings pull payload bytes through a chunk-returning stream:
gomobile copies a []byte argument into a fresh Java array and never copies
it back, so a fill-my-buffer method would hand back the right length with
no data.

client/internal/peer/signaler.go

@@ -1,6 +1,8 @@
 package peer
 
 import (
+	"sync/atomic"
+
 	"github.com/pion/ice/v4"
 	log "github.com/sirupsen/logrus"
 	"golang.zx2c4.com/wireguard/wgctrl/wgtypes"
@@ -12,6 +14,7 @@ import (
 type Signaler struct {
 	signal       signal.Client
 	wgPrivateKey wgtypes.Key
+	filedropPort atomic.Uint32
 }
 
 func NewSignaler(signal signal.Client, wgPrivateKey wgtypes.Key) *Signaler {
@@ -44,6 +47,12 @@ func (s *Signaler) Ready() bool {
 	return s.signal.Ready()
 }
 
+// SetFiledropPort sets the file drop listen port advertised in offers and answers;
+// 0 means the well-known default and is not put on the wire.
+func (s *Signaler) SetFiledropPort(port uint16) {
+	s.filedropPort.Store(uint32(port))
+}
+
 // SignalOfferAnswer signals either an offer or an answer to remote peer
 func (s *Signaler) signalOfferAnswer(offerAnswer OfferAnswer, remoteKey string, bodyType sProto.Body_Type) error {
 	var sessionIDBytes []byte
@@ -57,6 +66,7 @@ func (s *Signaler) signalOfferAnswer(offerAnswer OfferAnswer, remoteKey string,
 	msg, err := signal.MarshalCredential(s.wgPrivateKey, remoteKey, signal.CredentialPayload{
 		Type:         bodyType,
 		WgListenPort: offerAnswer.WgListenPort,
+		FiledropPort: uint16(s.filedropPort.Load()),
 		Credential: &signal.Credential{
 			UFrag: offerAnswer.IceCredentials.UFrag,
 			Pwd:   offerAnswer.IceCredentials.Pwd,