feat: add scroll-to-top button on dashboard

[Rashi1404]

Description

Closes #2509

Adds a floating "Scroll to Top" button to the Dashboard that appears when the user scrolls past 400px. Clicking it smoothly scrolls back to the top.

Changes

• Created src/components/ScrollToTop.tsx
• Integrated into Dashboard layout
• Uses lucide-react ArrowUp icon
• Smooth scroll with window.scrollTo({ behavior: 'smooth' })
• Respects light/dark theme
• Accessible with aria-label and focus ring

Testing

• Button hidden at top of page
• Button appears after 400px scroll
• Click smoothly scrolls to top
• Works in light and dark mode
• No overlap with other floating elements

Checklist

• npm run lint passes
• npm run type-check passes
• Follows contributing guidelines

[github-actions]

GSSoC Label Checklist 🏷️

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• level:advanced — 55 pts
• level:critical — 80 pts

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• quality:exceptional — ×1.5 multiplier

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