fix(a11y): respect prefers-reduced-motion in BackToTopButton

[Shan7Usmani]

Summary

Adds prefers-reduced-motion support to the existing BackToTopButton component so users who prefer reduced animations get instant scroll behavior instead of smooth transitions.

Closes #3149

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 💥 Breaking change (fix or feature that changes existing behavior)
• 📝 Documentation update
• ♻️ Refactor / code cleanup (no functional change)
• ⚡ Performance improvement
• 🔒 Security fix
• 🧪 Tests only

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What Changed

• src/components/BackToTopButton.tsx — Added prefersReducedMotion check using window.matchMedia("(prefers-reduced-motion: reduce)"). Scroll behavior uses "auto" instead of "smooth" when reduced motion is preferred. Progress ring SVG transition is disabled via !transition-none Tailwind class for reduced motion users.

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How to Test

1. Open DevTrack dashboard and scroll down past 300px to reveal the Back to Top button
2. Click the button — should smooth scroll to top (default behavior)
3. Enable "Reduce motion" in OS accessibility settings (Windows: Settings > Accessibility > Visual effects > Animation effects; macOS: System Settings > Accessibility > Display > Reduce motion)
4. Scroll down again and click the button — should instantly jump to top without animation
5. Verify the progress ring around the button also snaps without transition

Expected result: Button scrolls smoothly by default, instantly jumps when reduced motion is preferred. No visual regressions on the button appearance or positioning.

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Checklist

• Linked the related issue above
• Self-reviewed my own diff
• No unnecessary console.log, debug code, or commented-out blocks
• npm run lint passes locally
• No TypeScript errors (npm run type-check)
• Added or updated tests where applicable
• Updated documentation / comments if behavior changed

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Accessibility (UI changes only)

• Keyboard navigation works correctly
• Color contrast meets WCAG AA standard
• ARIA labels / roles added where needed (existing aria-label="Back to top" preserved)
• Tested on mobile / responsive layout

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Additional Context

The BackToTopButton component already existed globally in providers.tsx and met all other acceptance criteria from the issue (300px threshold, smooth scroll, hidden near top, responsive, aria-label). This PR adds the missing prefers-reduced-motion a11y support.

[github-actions]

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