Add skip link for keyboard accessibility

[web3blind]

Summary

Adds a skip-to-content link so keyboard and screen-reader users can bypass repeated navigation and jump directly to the main content.

What changed

• Added a Skip to main content link in the root layout.
• The link is visually hidden until focused.
• Added id="main-content" and tabIndex={-1} to the main landmarks on key pages.

Why

This addresses #13 and supports WCAG 2.4.1 Bypass Blocks by giving users a reliable way to skip repeated navigation.

Validation

• Ran targeted ESLint on the changed files.
• No new errors were introduced in the changed files.
• Note: the full project lint currently reports pre-existing unrelated issues in other files.

[netlify]

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Name	Link
🔨 Latest commit	202096e