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>

main/esp_now_comm.h

@@ -4,6 +4,7 @@
 #include "app_config.h"
 #include "client_registry.h"
 #include "esp_err.h"
+#include "esp_now_messages.pb.h"
 #include "uart_messages.pb.h"
 
 esp_err_t esp_now_comm_init(const app_config_t *config);