Add software LiPo UV protection with energy-save boot.

Detect under-voltage on GPIO1/11, persist UV state in NVS, enter a minimal
power mode without ESP-NOW, and recover after charge; expose LED mode 6
battery-low for host testing via goTool.

Co-authored-by: Cursor <cursoragent@cursor.com>

main/board_input.h

@@ -19,6 +19,15 @@ typedef struct {
  */
 esp_err_t board_input_init(void);
 
+/** LiPo ADC channels only (no button, no background task). */
+esp_err_t board_input_init_adc_only(void);
+
+/** Start 10 s LiPo monitor task (normal boot only). */
+esp_err_t board_input_start_lipo_monitor(void);
+
+/** Front-panel button debounce task. */
+esp_err_t board_input_init_button(void);
+
 /** On-demand ADC read of both LiPo sense inputs (if configured). */
 void board_input_read_lipo(board_lipo_reading_t *out);