Add file listing: share-dir flag, file_list_req/resp DataChannel messages

- proto: FileEntry, FileListResp types; MsgFileListReq/Resp msg types;
  CmdGetFileList + EvtFileList IPC types; Files field on IpcMessage
- mesh: ShareDir field + ScanShareDir(); on DataChannel open, auto-send
  MsgFileListReq to new peer; handle MsgFileListReq (scan + reply) and
  MsgFileListResp (emit EvtFileList to IPC subscribers)
- ipc: get_file_list command — own list returned immediately; remote peer
  list requested via DataChannel (response arrives as EvtFileList event)
- daemon: -share-dir flag wired to mesh.ShareDir
- test scripts: pass -share-dir /home/frejoh/Downloads/{alice,bob,charlie};
  test-network.sh verifies each peer's own file list via get_file_list
- FUTURE.md: document per-network share directories and multi-network design

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

internal/mesh/peer.go

@@ -56,6 +56,11 @@ func WireDataChannel(
 		}
 		m.AddPeer(peerConn)
 
+		// Request the peer's file list immediately after connect.
+		if req, err := json.Marshal(proto.PeerMessage{Type: proto.MsgFileListReq}); err == nil {
+			sendCh <- req
+		}
+
 		go func() {
 			for payload := range sendCh {
 				if err := dc.SendText(string(payload)); err != nil {
@@ -147,6 +152,28 @@ func handleDCMessage(data []byte, from proto.PeerID, localID *crypto.Identity, m
 
 func dispatchPeerMessage(msg proto.PeerMessage, from proto.PeerID, m *Mesh) {
 	switch msg.Type {
+	case proto.MsgFileListReq:
+		// Peer wants our file list — reply directly on their send channel.
+		files := m.ScanShareDir()
+		resp, err := json.Marshal(proto.PeerMessage{
+			Type:         proto.MsgFileListResp,
+			FileListResp: &proto.FileListResp{Files: files},
+		})
+		if err != nil {
+			return
+		}
+		m.SendTo(from, resp)
+
+	case proto.MsgFileListResp:
+		// Received a remote peer's file list — forward to IPC subscribers.
+		if msg.FileListResp != nil {
+			m.Emit(proto.IpcMessage{
+				Type:   proto.EvtFileList,
+				PeerID: peerIDPtr(from),
+				Files:  msg.FileListResp.Files,
+			})
+		}
+
 	case proto.MsgChat:
 		if msg.Chat != nil {
 			m.SaveMessage(msg.Chat)