Make the paddle actually advance

game.ex

@spec advance_paddle(Socket.t()) :: Socket.t()
defp advance_paddle(%{assigns: %{paddle: paddle, unit: unit}} = socket) do
  case paddle.direction do
    :left -> assign(socket, :paddle, move_paddle_left(paddle, unit))
    :right -> assign(socket, :paddle, move_paddle_right(paddle, unit))
    :stationary -> socket
  end
end

@spec move_paddle_left(map(), number()) :: map()
defp move_paddle_left(paddle, unit) do
  new_left = max(unit, paddle.left - paddle.speed)

  %{paddle | left: new_left, right: paddle.right - (paddle.left - new_left)}
end

@spec move_paddle_right(map(), number()) :: map()
defp move_paddle_right(paddle, unit) do
  new_left = min(paddle.left + paddle.speed, unit * (@board_cols - paddle.length - 1))

  %{paddle | left: new_left, right: paddle.right + (new_left - paddle.left)}
end