fix: improve landing page mobile responsiveness

[orvalvera]

What changed

/claim #3

Improves the landing page responsiveness for small mobile, tablet, and desktop breakpoints. The patch keeps the change focused to frontend/src/pages/LandingPage.tsx.

• Prevents the 320px header actions from wrapping or crowding the brand.
• Tightens mobile hero spacing so the headline, copy, CTAs, and visual fit cleanly in the first viewport.
• Uses the existing hero asset on mobile/tablet instead of hiding the visual entirely.
• Makes hero and CTA buttons full-width on small screens, then restores inline layout on larger screens.
• Moves the tools grid from cramped two-column mobile cards to a safer 1-column / 2-column / 4-column progression.
• Adds reduced-motion handling for the reveal/prism animations.

Fixes #3

Verification

• npm run build
• npx eslint src/pages/LandingPage.tsx
• Visual/overflow checks at 320x800, 375x812, 768x1024, and 1440x900: no document-level horizontal overflow and nav text no longer wraps on 320px.

Notes

• npm run lint still fails on existing unrelated files outside this patch, including AdminUsersPage.tsx, SplitTool.tsx, PdfPasswordModal.tsx, PdfViewer.tsx, AnnotationCanvas.tsx, and ImageToPdfPanel.tsx.
• npm run test still fails in the existing localStorage test setup: localStorage.clear is not a function and storage.setItem is not a function.

[Copilot]

Pull request overview

This PR improves the LandingPage layout across small mobile, tablet, and desktop breakpoints to prevent header wrapping/overcrowding, make the hero content fit cleanly in the first viewport on small screens, and make the tools grid adapt more safely to narrow widths.

Changes:

• Adjusted nav spacing/typography and constrained flex behavior to avoid brand/action crowding at ~320px widths.
• Reworked hero layout/spacing and CTA button sizing for small screens, and introduced a consistent hero visual using the existing hero.png on mobile/tablet and desktop.
• Updated tools grid breakpoints to a 1 → 2 → 4 column progression and added prefers-reduced-motion handling for the reveal/fade/prism animations.

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