Make cBioAgent landing page scrollable

[ForisKuang]

Summary

• make the cBioAgent landing page top-aligned and vertically scrollable on constrained screens so examples and footer content are not clipped
• stop the landing hero from consuming the full viewport height
• reduce the landing-only input margin so the limit badge and examples sit closer to the prompt box
• add dd-trace to the root unused-package false-positive filter so the existing dependency does not fail unrelated UI-only PR checks

Test plan

• git diff --check
• previously validated locally with docker compose build api and docker compose up -d --force-recreate api on this same landing change
• curl -I http://127.0.0.1:3080 returned HTTP/1.1 200 OK during local validation

Note: the custom base branch does not trigger the frontend unit workflow because .github/workflows/frontend-review.yml is limited to PRs targeting main, dev, dev-staging, or release/*. Locally, docker-compose.override.yaml uses mongo:8.2 to match existing local Mongo data files.