Challenge containers: answers hidden in print/single-page version

[avivace]

Problem

The :::challenge custom container uses a <details>/<summary> HTML block to hide answers behind a collapsible element:

<details>
  <summary>Answer (Click me!)</summary>
  ...
</details>

Browsers do not expand <details> elements when printing, and the element is collapsed by default in the single-page version (single.html) as well. This means answers are completely inaccessible in both the printable and single-page versions of the tutorial, making those versions less useful for educators and offline readers.

Expected behaviour

Answers should be visible (i.e. the <details> element should be expanded) in the print page (print.html) and single-page version (single.html).

Possible fixes

• CSS: add a @media print rule to force <details> open when printing:

  @media print {
    details { display: block; }
    details summary { display: none; } /* hide the "Click me!" prompt */
  }

• Renderer post-processing: in renderer/src/main.rs, expand all <details> elements (add the open attribute) when post-processing print.html and single.html.
• Both: the CSS fix covers the print stylesheet; the renderer fix covers single.html.