docs(AC-28): elaborate ListBox.js comments and add AC-28.md (pre-assigned issue)

[nmahesh-cainc]

This pull request contains documentation-only changes meant to clarify behavior in test-app/src/widgets/ListBox.js and adds AC-28.md which serves as the pre-assigned issue for this change.

Summary of changes:

• Expanded inline comments in ListBox.js to explain:
  • Why the listbox container is reparented to the top when opening the dropdown.
  • How keyboard focus is moved into the drop-down for accessibility.
  • The visual arrow/chevron drawing is purely presentational.
  • Drop-down blur behavior that collapses the list and returns logical focus.
• Added AC-28.md (pre-assigned issue file) to track the change in the repository since GitHub Issues are disabled on this fork.

Acceptance criteria:

• All edits are comment-only; no executable logic changed.
• The repository builds and lint rules should remain unaffected.

See branch: feature/AC-28-assigning-tasks-to-gh-copilot