feat: Add scroll-to-top button component (#340)

- Add ScrollToTop component at frontend/src/components/common/ScrollToTop.tsx
- Button appears when scrolling down more than 300px
- Smooth scroll animation to top on click
- Fade-in/fade-out animations
- Dark theme styling with indigo-600
- Accessible with aria-label and keyboard focusable
- No new dependencies, uses CSS transitions

Closes #340

Author: unknown