feat: add skip link and main landmark

[Tiyatrotist]

Adds a keyboard-visible Skip to main content link and wraps the page's primary content in a main landmark, giving keyboard and screen-reader users a direct bypass around navigation. Closes #745.

Validation:

• scripts/check_em_dash.py: passed
• scripts/check_broken_links.py: passed (1655 links)
• git diff --check: passed
• HTML structure assertions: passed
• Full make lint could not complete because Ruff is not installed in the WSL environment; the Ruff step only targets Python paths and this PR changes index.html only.

[vercel]

@Tiyatrotist is attempting to deploy a commit to the yashkewlani2020-gmailcom's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

Hey @Tiyatrotist, your first PR on Fair Code, that's awesome, thank you.

This project is about making AI more accountable, and contributions like yours are what keep that work going. We'll review your changes shortly.

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[github-actions]

@yakew7 - new PR touching a path you own, please review.