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/proto/proto.go

@@ -225,6 +225,7 @@ const (
 	EvtError            IpcMsgType = "error"
 	EvtInviteGenerated  IpcMsgType = "invite_generated"
 	EvtFileList         IpcMsgType = "file_list"
+	EvtFileComplete     IpcMsgType = "file_complete"
 	EvtNetworkJoined    IpcMsgType = "network_joined"
 	EvtNetworkLeft      IpcMsgType = "network_left"
 )