Fix landing page mobile responsiveness

[brianrfdavis]

Fixes #3.

This updates the landing page responsive behavior for narrow/mobile viewports.

Changes:

• Add missing Tailwind brand colors used by existing CTA/nav classes.
• Prevent horizontal overflow on the landing page.
• Tighten mobile nav, hero, CTA, section, tools-grid, and footer spacing.
• Stack hero and CTA buttons on narrow screens.
• Make the tools grid work down to single-column mobile layouts.

Verification:

• npm run build passes in frontend/.
• npm run lint and npm test were checked locally but are blocked by unrelated pre-existing repo issues outside this landing-page fix.