Show cBioAgent examples by default

[ForisKuang]

Summary

• show categorized cBioAgent examples by default instead of hiding them behind the initial Show examples state
• preserve the existing capability-card layout and click-to-expand behavior
• make the landing page top-aligned and vertically scrollable on constrained screens so examples and footer content are not clipped
• reduce the landing-only input margin so the limit badge and examples sit closer to the prompt box

Test plan

• git diff --check
• docker compose build api completed successfully, including the production frontend build
• docker compose up -d --force-recreate api
• confirmed the running container includes the updated landing scroll classes and showExamples=true
• curl -I http://127.0.0.1:3080 returned HTTP/1.1 200 OK

Local test URL: http://localhost:3080

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 the existing local Mongo data files.