fn add_border(picture: &[String]) -> Vec<String> {
    let width = picture[0].len();
    let border = "*".repeat(width + 2);

    let mut result = Vec::with_capacity(picture.len() + 2);
    result.push(border.clone());
    for row in picture {
        result.push(format!("*{row}*"));
    }
    result.push(border);

    result
}