feat: Add Scroll-to-Top Button Component | Fixes #576

[Yugal0708]

📌 What does this PR do?

Adds a ScrollToTop reusable React component that improves navigation UX
for long pages (PYQs listings, syllabus pages, etc.).

✅ Changes Made

• Added client/src/components/ScrollToTop.jsx — functional component with scroll listener
• Added client/src/components/ScrollToTop.css — responsive styling
• Imported and rendered <ScrollToTop /> in App.jsx

🧠 How it works

• Uses useEffect to attach a scroll event listener
• Button becomes visible after scrolling 300px down
• On click, window.scrollTo({ top: 0, behavior: 'smooth' }) triggers smooth scroll
• Cleans up event listener on component unmount (no memory leaks)

📱 Tested on

• Desktop (Chrome)
• Mobile viewport (responsive)
• No console errors

🔗 Related Issue

Closes #576

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GSSoC 2025 Contributor | Made by @Yugal0708

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