Add a new UI route to serve the interactive demo interface at `/api/ui`. Replit-Commit-Author: Agent Replit-Commit-Session-Id: 418bf6f8-212b-4bb0-a7a5-8231a061da4e Replit-Commit-Checkpoint-Type: full_checkpoint Replit-Commit-Event-Id: 3fb69144-fc09-46cf-8560-9b4bc828c60f Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/9f85e954-647c-46a5-90a7-396e495a805a/418bf6f8-212b-4bb0-a7a5-8231a061da4e/sPDHkg8 Replit-Helium-Checkpoint-Created: true