feat: implement §9 file transfer over dedicated binary DataChannels

File chunks travel over a per-transfer "f:<xid>" WebRTC DataChannel —
direct DTLS-encrypted P2P, the anchor never sees file data. Once the
initial handshake is done the anchor can disappear and transfers continue.

Key design choices:
- Receiver sends "ok" on the file DC before sender streams, eliminating
  a race where tiny files could be fully sent/closed before the receiver's
  OnMessage handler is registered (open-race §6 analogue for data DCs).
- Auto-accept: receiver accepts every incoming offer immediately.
- Download dir: per-network at <data-dir>/downloads-<netid>/.
- Backpressure: bufferedAmountLowThreshold to avoid overwhelming the DC.
- SHA-256 verified on receive; mismatches emit EvtError and discard temp file.
- IPC: send_file {peer_id, path} → offers the named file from share dir.
- EvtFileComplete {transfer_id, path} emitted on success.

IPC command: {"type":"send_file","peer_id":"<hex>","path":"<filename>"}

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

internal/netmgr/manager.go

@@ -92,6 +92,7 @@ func (mgr *Manager) Join(name string) (string, error) {
 	if mgr.cfg.ShareDir != "" {
 		m.ShareDir = mgr.cfg.ShareDir
 	}
+	m.DownloadDir = filepath.Join(mgr.cfg.StoreDir, "downloads-"+netID_full)
 
 	// Forward all mesh events to the Manager's fan-out, tagging with network_id.
 	meshEvents := m.Subscribe()