Feature: add hover and press motion to landing buttons

[SandObserver]

Summary

• Fade background and text colour over 0.2s on the landing page pills and buttons, and fade link colour over 0.2s.
• Scale pills and buttons to 97% while pressed.
• Add a hover state to the solid Install pill in the nav, which had none.
• Drop every transition to 0.01ms under prefers-reduced-motion: reduce.

Testing
All green. Computed styles, forced hover and active states, and reduced motion checked in headless Chrome against the production preview, light and dark schemes.

Checklist

• npm run build and npm run check both pass.
• Product UI is shown by running or capturing it, never recreated in CSS.
• Any new preview or screenshot carries no private hostnames or real data.
• Visual changes verified in both colour schemes and both docs themes.
• Links to the app repo still resolve.

[cloudflare-workers-and-pages]

Deploying stackyard-docs with    Cloudflare Pages

Latest commit:	1ef739d
Status:	✅  Deploy successful!
Preview URL:	https://138b440c.stackyard-docs.pages.dev
Branch Preview URL:	https://feat-landing-button-motion.stackyard-docs.pages.dev

View logs