Add LiPo battery monitoring with ESP-NOW cache and dashboard API.

Slaves report pack voltages every 30s; the master caches them for fast
BATTERY_STATUS reads. goTool exposes REST/WebSocket and shows values in
the dashboard, with a nanopb fix so optional lipo submessages encode.

Co-authored-by: Cursor <cursoragent@cursor.com>

goTool/client_api.go

@@ -59,6 +59,86 @@ func (m *managedSerial) readAccelSnapshotPoll(clientID uint32) (*pb.AccelSnapsho
 	return decodeAccelSnapshotPayload(respPayload)
 }
 
+func decodeBatteryStatusPayload(payload []byte) (*pb.BatteryStatusResponse, error) {
+	if len(payload) < 2 {
+		return nil, fmt.Errorf("short battery response")
+	}
+	if payload[0] != byte(pb.MessageType_BATTERY_STATUS) {
+		return nil, fmt.Errorf("unexpected command id 0x%02x (want 0x%02x)",
+			payload[0], byte(pb.MessageType_BATTERY_STATUS))
+	}
+	var msg pb.UartMessage
+	if err := proto.Unmarshal(payload[1:], &msg); err != nil {
+		return nil, fmt.Errorf("decode: %w", err)
+	}
+	if msg.GetType() != pb.MessageType_BATTERY_STATUS {
+		return nil, fmt.Errorf("unexpected type %v", msg.GetType())
+	}
+	r := msg.GetBatteryStatusResponse()
+	if r == nil {
+		return nil, fmt.Errorf("missing battery_status_response")
+	}
+	return r, nil
+}
+
+func (m *managedSerial) BatteryStatus(req *pb.BatteryStatusRequest) (*pb.BatteryStatusResponse, error) {
+	var resp *pb.BatteryStatusResponse
+	err := m.withPort(func(sp *serialPort) error {
+		var e error
+		resp, e = sp.batteryStatus(req)
+		return e
+	})
+	return resp, err
+}
+
+func (m *managedSerial) BatteryStatusPoll(req *pb.BatteryStatusRequest) (*pb.BatteryStatusResponse, error) {
+	msg := &pb.UartMessage{
+		Type: pb.MessageType_BATTERY_STATUS,
+		Payload: &pb.UartMessage_BatteryStatusRequest{
+			BatteryStatusRequest: req,
+		},
+	}
+	body, err := proto.Marshal(msg)
+	if err != nil {
+		return nil, fmt.Errorf("encode: %w", err)
+	}
+	payload := append([]byte{byte(pb.MessageType_BATTERY_STATUS)}, body...)
+	respPayload, err := m.batteryStatusPayloadPoll(payload)
+	if err != nil {
+		return nil, err
+	}
+	return decodeBatteryStatusPayload(respPayload)
+}
+
+func (m *managedSerial) batteryStatusPayloadPoll(payload []byte) ([]byte, error) {
+	var resp []byte
+	err := m.withPortPoll(func(sp *serialPort) error {
+		var e error
+		resp, e = sp.exchangePayloadForBattery(payload, "BATTERY_STATUS")
+		return e
+	})
+	return resp, err
+}
+
+func (s *serialPort) batteryStatus(req *pb.BatteryStatusRequest) (*pb.BatteryStatusResponse, error) {
+	msg := &pb.UartMessage{
+		Type: pb.MessageType_BATTERY_STATUS,
+		Payload: &pb.UartMessage_BatteryStatusRequest{
+			BatteryStatusRequest: req,
+		},
+	}
+	body, err := proto.Marshal(msg)
+	if err != nil {
+		return nil, fmt.Errorf("encode: %w", err)
+	}
+	payload := append([]byte{byte(pb.MessageType_BATTERY_STATUS)}, body...)
+	respPayload, err := s.exchangePayloadForBattery(payload, "BATTERY_STATUS")
+	if err != nil {
+		return nil, err
+	}
+	return decodeBatteryStatusPayload(respPayload)
+}
+
 func (m *managedSerial) AccelStream(req *pb.AccelStreamRequest) (*pb.AccelStreamResponse, error) {
 	return m.accelStreamVia(m.withPort, req)
 }