Purchasing, Crewing and Administration headings are now collapsible
buttons (chevron + aria-expanded/aria-controls) that collapse by
default. Single-open accordion: opening one heading collapses any
other open one. The section containing the active route auto-expands
on mount/navigation so the user is never stranded on a hidden link.
Adds a jsdom/Testing Library unit test covering default-collapsed,
toggle, single-open accordion, and active-route auto-expand.
Fixes#96
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
