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/mesh.go

@@ -3,6 +3,7 @@ package mesh
 
 import (
 	"log"
+	"os"
 	"sync"
 
 	"github.com/waste-go/internal/crypto"
@@ -22,6 +23,7 @@ type PeerConn struct {
 type Mesh struct {
 	Identity *crypto.Identity
 	Store    *store.Store // may be nil if persistence is disabled
+	ShareDir string       // directory whose contents are shared with peers; "" = no sharing
 
 	mu    sync.RWMutex
 	peers map[proto.PeerID]*PeerConn
@@ -41,6 +43,31 @@ func New(id *crypto.Identity, st *store.Store) *Mesh {
 	}
 }
 
+// ScanShareDir returns the list of files in the local share directory.
+// Returns an empty slice if ShareDir is unset or the directory is empty.
+func (m *Mesh) ScanShareDir() []proto.FileEntry {
+	if m.ShareDir == "" {
+		return nil
+	}
+	entries, err := os.ReadDir(m.ShareDir)
+	if err != nil {
+		log.Printf("mesh: scan share dir %s: %v", m.ShareDir, err)
+		return nil
+	}
+	var files []proto.FileEntry
+	for _, e := range entries {
+		if e.IsDir() {
+			continue
+		}
+		info, err := e.Info()
+		if err != nil {
+			continue
+		}
+		files = append(files, proto.FileEntry{Name: e.Name(), SizeBytes: info.Size()})
+	}
+	return files
+}
+
 // ── Peer management ───────────────────────────────────────────────────────────
 
 // AddPeer registers a connected peer and notifies subscribers.