feat(cli): scaffold selected backend framework

[vishxrad]

Summary

• ask self-hosted CLI users to choose the default OpenAI SDK, LangGraph, or Vercel AI SDK backend
• keep one shared frontend template and replace only src/app/api/chat/route.ts for framework-specific implementations
• rewrite only the selected backend dependencies and remove route staging files from generated projects
• document the new flag and self-hosted backend choices

OpenUI Cloud remains on its managed backend because its client uses the Responses API and artifact event protocol.

Issue

TH-2051 — Ask for backend framework in CLI

Validation

• pnpm --filter @openuidev/cli build
• pnpm --filter @openuidev/cli lint:check
• pnpm --filter @openuidev/cli format:check
• verified default, LangGraph, and Vercel scaffolds differ only in package.json and src/app/api/chat/route.ts
• verified route staging directories are removed from generated projects
• installed, linted, and production-built generated LangGraph and Vercel applications