feat/mobile: Add overview navigation and quick jump support

- Add an Overview button for mobile devices.
- Include quick-jump navigation so users can directly jump to sections/pages instead of scrolling through the full track.

The issue names no files, tests, or entry points, so first locate the mobile navigation implementation in the website. Confirm the Overview button is available on mobile and that quick-jump controls reach the intended sections or pages without requiring full-track scrolling.

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