Fix focus management when opening the About modal

[coderabbitai]

Summary

When the About modal is opened via the menu, focus management has two related problems:

1. previouslyFocused captures #about-menu-item, which is inside the menu that becomes hidden immediately after — so returning focus on close may target a hidden element.
2. closeInfoModal.focus() targets a <span>, which is non-interactive by default, meaning keyboard focus may land outside the dialog. This means the Tab trap and Escape handler do not reliably engage for keyboard users.

Impact

Keyboard and assistive-technology users may lose focus context when opening or closing the About modal.

References

• PR: Tidy index #409
• Review comment: Tidy index #409 (comment)

Requested by @lawsie.