Added BackToTop button component

[KurimineniBhavanaSree]

Description

This PR adds a reusable Back To Top button component to improve navigation and overall user experience across the StudyMatePlus platform.

Currently, users have to manually scroll back to the top of long pages, which can feel inconvenient, especially on mobile devices or content-heavy pages. To solve this, I implemented a floating scroll-to-top button that becomes visible once the user scrolls down the page.

When clicked, the button smoothly scrolls the user back to the top of the page.

Changes Made

• Created a reusable BackToTop.js component inside the components folder
• Implemented scroll position detection using window.scrollY
• Added conditional rendering so the button only appears after scrolling down
• Added smooth scrolling functionality using window.scrollTo()
• Integrated the component globally through App.js so it works across all pages
• Styled the button with a clean circular UI and fixed positioning for better accessibility and usability
• Added proper cleanup for the scroll event listener using useEffect

Testing Done

• Verified that the button appears only after scrolling down
• Tested smooth scrolling behavior on click
• Checked functionality across multiple pages
• Ensured there are no console errors during rendering

Fixes #183

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[KurimineniBhavanaSree]

Kindly review my PR. Thank you!