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/serial_link.go

@@ -119,7 +119,7 @@ func (m *managedSerial) exchangePayloadVia(
 	var resp []byte
 	err := portFn(func(sp *serialPort) error {
 		var e error
-		resp, e = sp.exchangePayloadLocked(payload, cmdName)
+		resp, e = sp.exchangePayloadLocked(payload, cmdName, readTimeout)
 		return e
 	})
 	return resp, err