Fix landing page mobile responsiveness

[mindrica232-arch]

Summary

• Improves the landing page layout on narrow mobile screens.
• Prevents the nav actions from wrapping at 320px widths.
• Switches the tools grid to a single column on very small devices, then restores two/four columns at wider breakpoints.
• Centers CTA buttons and tightens mobile spacing across hero, toolkit, how-it-works, CTA, and footer sections.

Fixes #3.

Validation

• npm run build
• npx eslint src/pages/LandingPage.tsx
• Visually checked the landing page at 320px and 390px mobile widths.