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/autotest/runner.go

@@ -347,6 +347,8 @@ func ledRingModeValue(mode string) (uint32, error) {
 		return 3, nil
 	case "find_me", "findme":
 		return 4, nil
+	case "battery_low", "batterylow":
+		return 6, nil
 	case "color", "solid", "fill":
 		return 5, nil
 	default: