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/api_serve.go

@@ -70,6 +70,7 @@ type otaAPIResponse struct {
 func mountServeAPI(mux *http.ServeMux, link *managedSerial, hub *wsHub, streamCtl *accelStreamCtl) {
 	mountAccelStreamAPI(mux, link, hub, streamCtl)
 	mountLedRingAPI(mux, link)
+	mountBatteryAPI(mux, link)
 	mux.HandleFunc("/api/deadzone", func(w http.ResponseWriter, r *http.Request) {
 		switch r.Method {
 		case http.MethodGet: