a11y: add skip-to-main-content link in site layout

[ulises-jeremias]

Context

The CPA website has no "skip to main content" link. This is a small accessibility win for keyboard users.

Steps

1. Add a skip link as the first focusable element in the site shell/layout.
2. Ensure key pages expose id="main-content" (or equivalent) on <main>.
3. Style it visually hidden until focused; works in light and dark themes.

Acceptance criteria

• Skip link is first in tab order
• Activating it moves focus/viewport to main content
• Decorative-only; no layout shift when hidden
• Works on templates/extensions/docs pages that use the shared layout

Files

• src/components/layout-shell.tsx and/or src/app/layout.tsx
• Page <main> wrappers as needed

How to verify

Tab from the address bar into the page: first focus should be the skip link; Enter jumps to main.