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/powerpod.c

@@ -11,6 +11,7 @@
 #include "cmd_ota.h"
 #include "cmd_ota_slave_progress.h"
 #include "cmd_led_ring.h"
+#include "cmd_battery.h"
 #include "esp_now_comm.h"
 #include "powerpod.h"
 #include "driver/gpio.h"
@@ -163,6 +164,8 @@ void app_main(void) {
   ESP_LOGI(TAG, "Running Partition: %s (OTA slot %d)",
            app_config.running_partition, ota_slot);
 
+  board_input_init();
+
   err = esp_now_comm_init(&app_config);
   if (err != ESP_OK) {
     ESP_LOGE(TAG, "ESP-NOW init failed: %s", esp_err_to_name(err));
@@ -170,8 +173,6 @@ void app_main(void) {
 
   led_ring_init();
 
-  board_input_init();
-
   if (app_config.master) {
     cmd_queue = xQueueCreate(64, sizeof(generic_msg_t));
     init_cmdHandler(cmd_queue);
@@ -185,6 +186,7 @@ void app_main(void) {
     cmd_espnow_find_me_register();
     cmd_restart_register();
     cmd_led_ring_register();
+    cmd_battery_register();
     cmd_ota_register();
     cmd_ota_slave_progress_register();
   }