Update OpenUI Cloud ShadCN example

[AbhinRustagi]

What

Rebuild examples/shadcn-chat as the OpenUI Cloud reference example. Instead of calling OpenAI directly with a build-time-generated system prompt, the example now sends its shadcn component library to Thesys backend per-request via createResponsesInstructions({ componentLibrary }) — Thesys backend generates the component system prompt server-side, validates model output against the library's schema. The demo shows the full cloud surface: conversation threads, frontend tokens, artifacts (slides/report), server-side search tools, and an app-side function-tool loop.

Changes

• api/chat/route.ts — swapped chat-completions + runTools against OpenAI for the Responses API against Thesys API (/v1/embed) with conversation threads and store: true. Instructions carry the componentLibrary config block (shadcnLibraryConfig from lib/shadcn-genui/server).
• api/frontend-token/route.ts (new) — mints muse frontend tokens for the client SDK.
• page.tsx — AgentInterface wired with the cloud llm transport, artifact renderers/categories.
• .env.example (THESYS_API_KEY, OPENUI_MODEL, DEMO_USER_ID), lib/env.ts helpers; package.json gains @openuidev/thesys-server + openai.
• The build-time system-prompt.txt generation step is gone — the prompt is server-generated from the wire config.

Test Plan

• Verified locally

Checklist

• I linked a related issue, if applicable (TH-2035)
• I updated docs/README when needed (example README needs a pass: cloud env vars replace OPENAI_API_KEY + the prompt-generation step)
• I considered backwards compatibility