feat: add back to top button

[surbhiag1]

What changed

Related issue

Type of change

• Bug fix
• Feature
• Refactor (no behaviour change)
• Docs / comments only
• CI / tooling

Area affected

• Backend (Express / Mongoose)
• Frontend (React / Vite)
• Admin / Train tab (/admin/*)
• Community (/community — posts, comments, auto-answer)
• Search (hybrid text retrieval, training stats)
• Auth / middleware / samagama.in bridge
• Crons / schedulers / embedding-warm
• Observability (Sentry / logging / Discord alerts)
• Docs

CI verification

• cd apps/backend && npx tsc --noEmit exits 0
• cd apps/backend && npx vitest run — all tests pass
• cd apps/frontend && npx tsc --noEmit exits 0
• cd apps/frontend && npx vitest run — all tests pass
• pnpm run lint — 0 errors (152 warnings is the baseline)
• GitHub Actions green on the merge commit (CI, CodeQL, Build & Deploy)
• Tested with a real API hit or browser interaction if behaviour changed
• Tests added or updated for the change
• Single logical change — unrelated fixes noted in description, not fixed here
• Docs updated if route / API / env var / pipeline behaviour changed
• Rebased onto main, no merge commits

Notes for reviewer

[surbhiag1]

Feature Added: Back-to-Top Button

Added a Back-to-Top feature to the website so users can quickly return to the top of the page after scrolling.

Work completed:

• Created a new "BackToTop" component in:
  "apps/frontend/src/components/ui/BackToTop.tsx"
• Added the "BackToTop" component to:
  "apps/frontend/src/App.tsx"
• Implemented the button to appear when the user scrolls down the page.
• Added functionality to smoothly scroll the user back to the top when the button is clicked.
• Tested the feature on the local development website and confirmed that the arrow button appears and works correctly.

Feature location:
"apps/frontend/src/components/ui/BackToTop.tsx"

Integration:
"apps/frontend/src/App.tsx"