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/board_input.h

@@ -1,12 +1,25 @@
 #ifndef BOARD_INPUT_H
 #define BOARD_INPUT_H
 
+#include <stdbool.h>
+#include <stdint.h>
+
 #include "esp_err.h"
 
+typedef struct {
+  bool lipo1_valid;
+  bool lipo2_valid;
+  uint32_t lipo1_mv;
+  uint32_t lipo2_mv;
+} board_lipo_reading_t;
+
 /**
- * Button (log on press) and LiPo ADC sampling (log every 10 s).
+ * Button (log on press) and LiPo ADC sampling (background log every 10 s).
  * TODO: Pin assignments come from powerpod.h and may not match final hardware yet.
  */
 esp_err_t board_input_init(void);
 
+/** On-demand ADC read of both LiPo sense inputs (if configured). */
+void board_input_read_lipo(board_lipo_reading_t *out);
+
 #endif