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/cmd/cmd_led_ring.c

@@ -13,6 +13,7 @@ static const char *TAG = "[LED_RING_CMD]";
 #define LED_RING_MODE_BLINK    3
 #define LED_RING_MODE_FIND_ME  4
 #define LED_RING_MODE_COLOR    5
+#define LED_RING_MODE_BATTERY_LOW 6
 
 static uint8_t clamp_u8(uint32_t v) {
   if (v > 255) {
@@ -90,6 +91,10 @@ bool cmd_led_ring_apply(const alox_LedRingProgressRequest *req) {
     led_ring_find_me();
     return true;
 
+  case LED_RING_MODE_BATTERY_LOW:
+    led_ring_show_battery_low();
+    return true;
+
   case LED_RING_MODE_BLINK:
     cmd.mode = LED_CMD_BLINK;
     cmd.r = r;