fix(core/structure): improve "back to top" link accessibility

[Copilot]

Accessibility audits flagged the "back to top" link as too small/hard to click and misusing role="navigation" on a single link rather than a group of navigation links.

Markup

• Removed role="navigation" from the #back-to-top element.
• Replaced the icon-only <abbr title="..."> (accessible name hidden in a title attribute) with visible link text plus a decorative, aria-hidden="true" arrow:

<!-- before -->
<p role="navigation" id="back-to-top">
  <a href="#title"><abbr title="Back to Top">↑</abbr></a>
</p>

<!-- after -->
<p id="back-to-top">
  <a href="#title"><span aria-hidden="true">↑</span> Back to Top</a>
</p>

Styling

• Added padding to #back-to-top a for a larger, easier-to-click/tap target.
• Hid #back-to-top in print media, since it has no purpose on paper.

Tests

• Updated existing structure-spec.js tests (title link, French localization) to match the new markup.
• Added a test asserting role="navigation" is absent and that the link exposes visible accessible text.

• Fixes Accessibility improvement : "back to top" icon #5337