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/ipc/ipc.go

@@ -198,6 +198,26 @@ func handleClient(conn net.Conn, m *mesh.Mesh, anchorURL string, currentNetwork
 				Rooms:          []string{"general"},
 			})
 
+		case proto.CmdGetFileList:
+			if cmd.PeerID == nil || *cmd.PeerID == m.Identity.PeerID() {
+				// Local list — scan immediately.
+				send(proto.IpcMessage{
+					Type:   proto.EvtFileList,
+					PeerID: ptr(m.Identity.PeerID()),
+					Files:  m.ScanShareDir(),
+				})
+			} else {
+				// Remote list — send a request over the DataChannel.
+				// The response arrives as EvtFileList via the mesh event bus.
+				req, err := json.Marshal(proto.PeerMessage{Type: proto.MsgFileListReq})
+				if err != nil {
+					continue
+				}
+				if !m.SendTo(*cmd.PeerID, req) {
+					send(errMsg(fmt.Sprintf("get_file_list: peer %s not connected", (*cmd.PeerID).Short())))
+				}
+			}
+
 		case proto.CmdGenerateInvite:
 			net := currentNetwork()
 			if net == "" {