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/proto/proto.go

@@ -34,13 +34,15 @@ type PeerInfo struct {
 type MsgType string
 
 const (
-	MsgChat       MsgType = "chat"
-	MsgPeerGossip MsgType = "peer_gossip"
-	MsgFileOffer  MsgType = "file_offer"
-	MsgFileResp   MsgType = "file_response"
-	MsgFileDone   MsgType = "file_done"
-	MsgPing       MsgType = "ping"
-	MsgPong       MsgType = "pong"
+	MsgChat         MsgType = "chat"
+	MsgPeerGossip   MsgType = "peer_gossip"
+	MsgFileListReq  MsgType = "file_list_req"
+	MsgFileListResp MsgType = "file_list_resp"
+	MsgFileOffer    MsgType = "file_offer"
+	MsgFileResp     MsgType = "file_response"
+	MsgFileDone     MsgType = "file_done"
+	MsgPing         MsgType = "ping"
+	MsgPong         MsgType = "pong"
 )
 
 // PeerMessage is the top-level container sent over the "yaw" DataChannel.
@@ -49,12 +51,13 @@ type PeerMessage struct {
 	Type MsgType `json:"type"`
 
 	// Only one of these will be set, depending on Type.
-	Chat      *ChatMessage  `json:"chat,omitempty"`
-	Gossip    *PeerGossip   `json:"gossip,omitempty"`
-	FileOffer *FileOffer    `json:"file_offer,omitempty"`
-	FileResp  *FileResponse `json:"file_response,omitempty"`
-	FileDone  *FileDone     `json:"file_done,omitempty"`
-	Seq       *uint64       `json:"seq,omitempty"` // for ping/pong
+	Chat         *ChatMessage  `json:"chat,omitempty"`
+	Gossip       *PeerGossip   `json:"gossip,omitempty"`
+	FileListResp *FileListResp `json:"file_list_resp,omitempty"`
+	FileOffer    *FileOffer    `json:"file_offer,omitempty"`
+	FileResp     *FileResponse `json:"file_response,omitempty"`
+	FileDone     *FileDone     `json:"file_done,omitempty"`
+	Seq          *uint64       `json:"seq,omitempty"` // for ping/pong
 }
 
 // ChatMessage is a message to a room or a DM.
@@ -80,6 +83,18 @@ type GossipEntry struct {
 	LastSeen time.Time `json:"last_seen"`
 }
 
+// FileEntry describes a single file in a peer's shared directory.
+type FileEntry struct {
+	Name      string `json:"name"`
+	SizeBytes int64  `json:"size_bytes"`
+}
+
+// FileListResp is the payload for MsgFileListResp.
+// MsgFileListReq carries no payload — it is a zero-field request.
+type FileListResp struct {
+	Files []FileEntry `json:"files"`
+}
+
 // FileOffer initiates a file transfer.
 type FileOffer struct {
 	Mid       string `json:"mid"`        // dedup id
@@ -187,6 +202,7 @@ const (
 	CmdGetState     IpcMsgType = "get_state"
 	CmdSendFile      IpcMsgType = "send_file"
 	CmdGenerateInvite IpcMsgType = "generate_invite"
+	CmdGetFileList    IpcMsgType = "get_file_list"
 
 	// Events (daemon → UI)
 	EvtMessageReceived  IpcMsgType = "message_received"
@@ -198,6 +214,7 @@ const (
 	EvtStateSnapshot    IpcMsgType = "state_snapshot"
 	EvtError            IpcMsgType = "error"
 	EvtInviteGenerated  IpcMsgType = "invite_generated"
+	EvtFileList         IpcMsgType = "file_list"
 )
 
 // IpcMessage covers both commands and events.
@@ -229,4 +246,5 @@ type IpcMessage struct {
 	Rooms          []string     `json:"rooms,omitempty"`
 	ErrorMessage   string       `json:"error_message,omitempty"`
 	InviteString   string       `json:"invite,omitempty"`
+	Files          []FileEntry  `json:"files,omitempty"`
 }