fix: make landing page responsive on mobile

[wangedmund77-cmyk]

/claim #3

Summary

• Tightens mobile nav spacing and prevents the primary CTA from wrapping awkwardly.
• Uses fixed breakpoint typography instead of viewport-scaled clamp classes for landing-page headings.
• Stacks hero/CTA buttons full-width on mobile, switches the tool grid to one column below 420px, and reduces section padding on small screens.
• Centers footer content on mobile while preserving desktop layout.

Verification

• npm ci
• npm run build (passes; Vite reports an existing large chunk warning)

[wangedmund77-cmyk]

Pushed a focused validation follow-up for the mobile responsiveness bounty.

What changed:

• Added src/pages/LandingPage.test.tsx regression coverage for the narrow-phone layout decisions: non-wrapping nav CTA, full-width mobile hero/final CTAs, and the grid-cols-1 -> min-[420px]:grid-cols-2 -> sm:grid-cols-4 tools grid.
• Updated the existing apiFetch test expectation to include the implementation's credentials: 'include', so the full frontend test suite is clean.

Verification:

• npm test -> 6 files / 63 tests passed.
• npm run build -> passed; Vite still reports the existing large chunk warning.
• git diff --check -> clean.