fix: harden landing page mobile responsiveness

[Swelm689]

Closes #3

/claim #3

Summary

• prevent the mobile header from wrapping at narrow widths while preserving the sign-in path in the hero
• stack both CTA groups into full-width touch targets below 420px
• use compact one-column tool rows below 380px, then scale to two and four columns
• tighten mobile spacing and typography without changing the desktop composition
• prevent decorative layers from affecting horizontal layout or full-page rendering
• add a reduced-motion mode for the landing-page animations

Validation

• npm run build — passes
• targeted ESLint for LandingPage.tsx — passes
• verified at 320px, 360px, 375px, 390px, and 768px
• confirmed scrollWidth === clientWidth at every tested breakpoint

Repository-wide lint and test commands still report pre-existing failures outside this change. The production build and the modified file both pass.