Main nav accessibility improvements

[acdvs]

Improves the accessibility of the main nav.

• Basic keyboard navigation
• Screen reader-friendly labeling

The Tooltip component now passes its generated unique label for use by any child element that needs to attach it (e.g., for aria-labelledby).

Ideally, the entire header nav needs to be restructured to make better use of semantic HTML and to standardize button structure, but I didn't want to interfere with any styling for this PR.

[CLAassistant]

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