Fix 320px landing-page navigation overflow

[MyTH-zyxeon]

Summary

• keep the 320px navigation inside the viewport without hiding the primary registration action
• collapse Sign in to an accessible icon below 380px and preserve the text label above that breakpoint
• enforce 44px navigation touch targets and stack hero/CTA actions on narrow screens

This is intentionally narrower than the earlier responsive rewrites: the current main already adapts most sections, while its brand + Sign in + Get started row still overflows at 320px.

Validation

• npx eslint src/pages/LandingPage.tsx
• npx vite build
• browser checks at 320x568, 375x812, and 768x1024: scrollWidth == clientWidth, all 8 tool cards remain in view, and navigation targets are at least 44px high
• browser console: no errors or warnings

The repository-wide npm run build remains blocked by pre-existing @testing-library/react type errors in the two overlay tests. The full test run also has pre-existing test-environment failures involving @testing-library/dom and localStorage; neither failure is in the changed file.

/claim #3

Run-Id: run-20260805T122344Z-misa3-crystal-pdf-mobile
Trace-Id: fd73bd78-8b18-4377-b2f3-2d06dc32a47d
Requester: Jun Discord sender_id 473730953735438336
Implementer: MISA 3 bot ID 1516725819517567077