UX: Add an accessible name to TopBanner's scroll control #417

Problem

The reusable hero scroll button is icon-only and has no accessible name.

Location

src/components/TopBanner.jsx, ScrollDownButton.

Impact

This affects Learn, Catalog, and other pages using TopBanner.

Expected

The button is named from its destination, for example "Scroll to Learn content." Decorative chevrons are hidden from assistive technology.

Acceptance criteria

• ScrollDownButton has a meaningful aria-label.
• The label can be derived from a human-readable prop rather than an element ID.
• Both chevron icons are aria-hidden.
• Keyboard activation moves to the intended section and does not strand focus.