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/cmd_serve.go

@@ -41,6 +41,7 @@ func runServe(portName string, baud int, args []string) error {
 	stop := make(chan struct{})
 	defer close(stop)
 	go runPoller(link, portName, hub, streamCtl, *interval, stop)
+	go runBatteryPoller(link, hub, 5*time.Second, stop)
 	go runAccelDashboardPoller(link, hub, *accelInterval, stop)
 
 	var apiSrv *http.Server