fix: handle hosted demo credit-limit errors

[ProfTrader]

Summary

Improves the hosted demo credit-limit handling so OpenRouter credit/rate-limit failures show the existing friendly demo-credits dialog instead of surfacing as a generic server error.

Related issue

Fixes #514

Changes

• Keep the existing HTTP 402 handling.
• Treat HTTP 429 provider responses as demo-credit exhaustion for the hosted demos.
• Detect nested provider messages mentioning credits, quota, rate limits, too many requests, or insufficient funds.
• Reuse the existing createDemoCreditsExhaustedResponse() payload so both playground and GitHub demo routes benefit without route-specific duplication.

Testing

• pnpm --filter @openuidev/docs run types:check
• pnpm exec prettier --check docs/lib/demo-credits.ts
• git diff --check