Demo showcase branch for localhost:3001

[ColinLi98]

Summary

• add a dedicated demo mode and seed/scenario data for the localhost:3001 showcase
• update the demo-facing pages and navigation for the standalone demo flow
• keep these demo-only changes isolated from the real-data presentation track

Validation

• pnpm typecheck
• pnpm test

Demo deployment

• https://promotion-agent-demo.vercel.app

Notes

• this PR is intended to track the standalone demo version separately from the real-data version
• keep it unmerged until we explicitly want these demo changes on main

[github-actions]

Vercel preview is ready: https://promotion-agent-ch7ly5dk2-songyis-projects-4d8a50e2.vercel.app