Improve landing page mobile responsiveness

[codex-bounty-e792cf]

/claim #3

Closes #3.

Summary

• Keeps the mobile nav on one line at 320px by tightening spacing, preserving the logo, and preventing auth buttons from wrapping.
• Adds a page-level horizontal overflow guard and mobile-first spacing for the hero, toolkit, how-it-works, CTA, and footer sections.
• Makes hero/CTA actions full-width on phones while preserving compact desktop buttons.
• Improves the tools grid so 320px devices use readable single-column cards, 360px+ phones use two columns, and tablet/desktop keep four columns.
• Replaces viewport-scaled headline sizing with Tailwind breakpoint sizes for steadier mobile typography.

Verification

• npm run build passes.
• npx eslint src/pages/LandingPage.tsx passes.
• Browser viewport checks after the fix:
  • 320px: overflowX = 0; nav links stay one line (Sign in 57px, Get started 92px).
  • 375px: overflowX = 0; hero buttons are full-width; tools grid renders two columns.
  • 768px and 1280px: overflowX = 0; tools grid renders four columns and desktop button sizing is preserved.

Note: npm run lint still reports pre-existing unrelated errors in admin/workspace/tool files outside this landing page change.

[codex-bounty-e792cf]

/claim #3

Claiming this Opire reward for PR #69. The fix keeps the landing page responsive at 320, 375, 768, and 1280 px; npm run build and npx eslint src/pages/LandingPage.tsx pass.