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>

goTool/led_ring_api.go

@@ -14,6 +14,7 @@ const (
 	ledRingModeBlink    = 3
 	ledRingModeFindMe   = 4
 	ledRingModeColor    = 5
+	ledRingModeBatteryLow = 6
 )
 
 type ledRingAPIRequest struct {
@@ -56,8 +57,10 @@ func ledRingModeFromString(s string) (uint32, error) {
 		return ledRingModeBlink, nil
 	case "find-me", "find_me", "findme":
 		return ledRingModeFindMe, nil
+	case "battery-low", "battery_low", "batterylow":
+		return ledRingModeBatteryLow, nil
 	default:
-		return 0, fmt.Errorf("unknown mode %q (clear, color, progress, digit, blink, find-me)", s)
+		return 0, fmt.Errorf("unknown mode %q (clear, color, progress, digit, blink, find-me, battery-low)", s)
 	}
 }