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/CMakeLists.txt

@@ -25,6 +25,7 @@ idf_component_register(
   "cmd/cmd_restart.c"
   "pod_reboot.c"
   "cmd/cmd_led_ring.c"
+  "cmd/cmd_battery.c"
   "cmd/cmd_ota.c"
   "cmd/cmd_ota_slave_progress.c"
   "ota_uart.c"