fix: polish landing page responsiveness on mobile

[maksiph14-glitch]

What changed

• Prevent horizontal overflow on narrow mobile viewports and allow the hero copy to wrap safely.
• Make the navigation fit 320px screens while keeping links at least 44px tall.
• Stack hero and final CTA actions on phones, with full-width tap targets.
• Show a fluidly sized crystal visual on mobile instead of removing it below md.
• Switch hover transitions to explicit properties and respect prefers-reduced-motion.
• Update the API test expectation for the existing cookie-auth credentials: include behavior.

Validation in GitHub Codespaces

Codespace: bookish-space-capybara-x5jq57gpgvv53v9pr

• npm ci ✅
• npm run build ✅
• npm test -- --run ✅ — 5 files, 60 tests
• npx eslint src/pages/LandingPage.tsx src/lib/api.test.ts ✅
• npm run lint still reports pre-existing errors in unrelated workspace/admin components; this PR does not touch those files.

Fixes #3

/claim #3