Issue 004: Mock chat MVP

[cody257]

Summary

• Adds a no-backend mock negotiation chat for seller and buyer modes.
• Replaces the seller one-shot form screen with a chat panel and mock assistant replies.
• Reuses the existing template generator for seller mock drafts.
• Adds Playwright e2e coverage for seller and buyer chat flows.
• Updates CI to run e2e tests.

Verification

• npm run lint
• npm test
• npm run test:e2e
• npm run build
• npm audit --audit-level=moderate

Notes

• No API route, model key, auth, database, or paywall added.
• Mock replies are intentionally simple so Issue 008 can replace the response source later.