fix: improve landing page mobile responsiveness

[1275321348]

Closes #3

Summary

• Tightened the mobile header spacing so nav actions do not wrap on narrow screens.
• Adjusted landing page section padding for small viewports.
• Stacked hero CTA buttons on very narrow screens and centered button content.
• Changed the tools grid to a one-column layout below 380px to avoid cramped cards.
• Made the final CTA button full-width on the narrowest screens.

Validation

• Verified 320px and 390px mobile widths with local screenshots.
• Confirmed no horizontal overflow at tested mobile widths.
• Ran pnpm exec tsc -b.
• Ran pnpm exec vite build.

Notes

This PR is intentionally limited to the landing page responsiveness issue.