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_stream.go

@@ -133,7 +133,8 @@ func (h *accelStreamHub) register(conn *websocket.Conn, portName string) *wsSubs
 		Serial:     portName,
 		IntervalMs: int(h.defaultInterval / time.Millisecond),
 		Commands: []string{
-			"set_stream", "get_stream", "set_accel_stream", "get_accel_stream", "set_led_ring",
+			"set_stream", "get_stream", "set_accel_stream", "get_accel_stream",
+			"set_led_ring", "get_battery",
 		},
 	}
 	if data, err := json.Marshal(hello); err == nil {
@@ -319,6 +320,15 @@ func writeStreamStatus(conn *websocket.Conn, msg StreamStatusMessage) {
 	_ = conn.WriteMessage(websocket.TextMessage, data)
 }
 
+func writeBatteryStatus(conn *websocket.Conn, out batteryAPIResponse) {
+	out.Type = "battery_status"
+	data, err := json.Marshal(out)
+	if err != nil {
+		return
+	}
+	_ = conn.WriteMessage(websocket.TextMessage, data)
+}
+
 func writeLedRingStatus(conn *websocket.Conn, out ledRingAPIResponse) {
 	out.Type = "led_ring_status"
 	data, err := json.Marshal(out)
@@ -416,10 +426,21 @@ func handleAccelWSCommand(conn *websocket.Conn, sub *wsSubscriber, data []byte,
 		}
 		writeLedRingStatus(conn, applyLedRing(link, body))
 
+	case "get_battery":
+		var body batteryAPIRequest
+		if err := json.Unmarshal(data, &body); err != nil {
+			writeBatteryStatus(conn, batteryAPIResponse{Error: "invalid JSON"})
+			return
+		}
+		if !body.AllClients && body.ClientID == 0 {
+			body.AllClients = true
+		}
+		writeBatteryStatus(conn, applyBatteryStatus(link, body))
+
 	default:
 		writeStreamStatus(conn, StreamStatusMessage{
 			Type: "stream_status",
-			Error: "unknown type (set_stream, get_stream, set_accel_stream, get_accel_stream, set_led_ring)",
+			Error: "unknown type (set_stream, get_stream, set_accel_stream, get_accel_stream, set_led_ring, get_battery)",
 		})
 	}
 }
@@ -478,6 +499,7 @@ func runAPIServer(portName string, link *managedSerial, addr string, defaultInte
 	mux := http.NewServeMux()
 	mountExternalAPI(mux, portName, defaultInterval, hub, link, dash, ctl)
 	mountLedRingAPI(mux, link)
+	mountBatteryAPI(mux, link)
 
 	srv := &http.Server{Addr: addr, Handler: mux}
 	go func() {