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/serialport.go

@@ -12,6 +12,9 @@ import (
 
 const readTimeout = 3 * time.Second
 
+// batteryReadTimeout: master may query each slave over ESP-NOW (~400 ms each).
+const batteryReadTimeout = 12 * time.Second
+
 type serialPort struct {
 	port  serial.Port
 	mu    sync.Mutex
@@ -44,10 +47,16 @@ func (s *serialPort) Close() error {
 func (s *serialPort) exchangePayload(payload []byte, cmdName string) ([]byte, error) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
-	return s.exchangePayloadLocked(payload, cmdName)
+	return s.exchangePayloadLocked(payload, cmdName, readTimeout)
 }
 
-func (s *serialPort) exchangePayloadLocked(payload []byte, cmdName string) ([]byte, error) {
+func (s *serialPort) exchangePayloadForBattery(payload []byte, cmdName string) ([]byte, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.exchangePayloadLocked(payload, cmdName, batteryReadTimeout)
+}
+
+func (s *serialPort) exchangePayloadLocked(payload []byte, cmdName string, timeout time.Duration) ([]byte, error) {
 	if len(payload) == 0 {
 		return nil, fmt.Errorf("empty payload")
 	}
@@ -63,6 +72,14 @@ func (s *serialPort) exchangePayloadLocked(payload []byte, cmdName string) ([]by
 		return nil, fmt.Errorf("write: %w", err)
 	}
 
+	if timeout <= 0 {
+		timeout = readTimeout
+	}
+	if err := s.port.SetReadTimeout(timeout); err != nil {
+		return nil, fmt.Errorf("set read timeout: %w", err)
+	}
+	defer func() { _ = s.port.SetReadTimeout(readTimeout) }()
+
 	respPayload, err := uartframe.ReadFrame(s.port, nil)
 	if err != nil {
 		return nil, fmt.Errorf("read response: %w", err)