feat(ux): smooth fade and polished states for scroll-to-top button

[singhanurag0317-bit]

Closes #933

What

The global scroll-to-top button (ScrollToTopButton, rendered in App.jsx) appeared/disappeared abruptly — it returned null when hidden — and lacked polished hover, active, focus, and dark-theme states.

Changes

• frontend/src/components/ScrollToTopButton.jsx
  • Button now stays in the DOM and toggles a .show class, so it fades in/out smoothly based on scroll position instead of popping in.
  • Uses a ChevronUp icon with aria-hidden for a cleaner look.
• frontend/src/components/ScrollToTopButton.css
  • Fade/scale/visibility transitions for smooth appearance and disappearance.
  • Polished :hover (lift + stronger shadow) and :active (press-down scale) states.
  • :focus-visible outline for keyboard accessibility.
  • Theme-aware colors using --color-primary tokens plus a [data-theme='dark'] gold variant for proper contrast.
  • Mobile sizing and a prefers-reduced-motion guard.
• Added ScrollToTopButton.test.jsx (4 tests): hidden at top, fades in on scroll, fades out at top, smooth scroll on click.

Verification

• npx vitest run src/test/__tests__/ScrollToTopButton.test.jsx — 4 passed.

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