Refactor presentations and add timeline UI

Convert presentations data to a flat, newest-first list (with year, month, month_name, etc.) and update the About page to render a grouped presentation timeline using Liquid's group_by. Add CSS for the presentation timeline (markers, year labels, entry meta, date badges) including light/dark variants. Also rename the heading "Work History" to "Selected Work History" to reflect the change.

Author: aaronparker