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/proto/uart_messages.pb.h

@@ -256,7 +256,7 @@ typedef struct _alox_EspNowEchoPingResponse {
 } alox_EspNowEchoPingResponse;
 
 /* Host → master: LED ring on master (client_id=0) and/or slaves via ESP-NOW.
- mode: 0=clear, 1=progress (0–100 %), 2=digit (0–10), 3=blink, 4=find-me, 5=all LEDs solid color. */
+ mode: 0=clear, 1=progress (0–100 %), 2=digit (0–10), 3=blink, 4=find-me, 5=all LEDs solid color, 6=battery-low. */
 typedef struct _alox_LedRingProgressRequest {
     uint32_t mode;
     /* * 0–100: fraction of ring LEDs to light (mode=progress) */