Add an HTMX copy handler for Copy link items in the social media menu template

Problem/Motivation

Drupal core 11.4.10 strips hx-* and on* attributes from menu link content, so a Copy link control cannot carry its HTMX handler in the menu link itself. Vartheme BS5 Rightup solved this in the theme (#3631079). Vartheme BS5 has the same nav-menu component and Social media menu template, so it should offer the same.

Proposed resolution

• menu--social-media-menu.html.twig adds hx-on:click (copy the page address, add is-copied, announce "Link copied") and hx-on:keydown (Space) to a menu link with the copy-link class, and attaches core/drupal.htmx only then. The message is translatable.
• The nav-menu SDC gains an optional per-item link_attributes, merged into the link.
• README: hx-on uses eval, so a strict Content Security Policy needs 'unsafe-eval' in script-src.

Nothing changes on a site until a Social media menu link has the copy-link class. Varbase Starter adds such a link in a separate issue.

Remaining tasks

• ✅ File an issue
• ✅ Addition/Change/Update/Fix
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support
• ➖ User Guide Documentation support
• ➖ UX/UI designer responsibilities
• ❌ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• None until a menu link has the copy-link class.

API changes

• Optional link_attributes per item in the nav-menu component.

Data model changes

• N/A

Release notes snippet

• A Social media menu link with the copy-link class copies the page address through core's HTMX.

AI-Generated: Yes. Drafted with Claude Code and reviewed before posting.