Make it easier to find "Sign Up" on mobile

[Lilly (Lillian)]

a pretty simple sidebar solution is to just add the link then hide it for everyone except mobile anon users using some simple css:

@use "lib/viewport";

.sidebar-section-link-wrapper[data-list-item-name="Sign up"] {
  display: none;

  .anon & {
    @include viewport.until(md) {
      display: block;
    }
  }
}