VS88 – Archive collection tree rollover test

Basis: VS87

Changed:
- archive/index.html
- shared/archive-core/archive-app.js
- shared/archive-core/archive-styles.css

Purpose:
- Keep the quick collections bar short.
- Show collection tree children as rollover/focus submenu.
- Make "Sammlungen" / "Collections" clickable and keyboard accessible as reset to the full archive collection view.

Not changed:
- no image data
- no captions
- no project folders
- no archive data
- no assets
- no pairings