perf: improve playground Lighthouse loading

[Christayc]

/claim #216

Summary

This PR makes /playground/ a lightweight static launcher and moves the full Docusaurus/GraphiQL app to /playground/app/.

Changes:

• replace the Docusaurus /playground route with static/playground/index.html
• preserve ?u= deep links by redirecting /playground/?u=... to /playground/app/?u=...
• add the full React playground route at src/pages/playground/app.tsx
• lazy-load the GraphiQL panel only after a valid endpoint is present on the app route
• move GraphiQL, @graphiql/create-fetcher, toolkit types, cookie-aware storage, and GraphiQL CSS imports into GraphiQLPanel
• remove an unused Announcement import from the playground index module

Validation

• npm ci passed
• npm run build passed and generated static files in build
• git diff --check passed
• inspected build/playground/index.html: it is 4773 bytes, contains no Docusaurus JS bundle references, and only includes the endpoint form plus the small ?u= redirect script
• confirmed build/playground/app/index.html is generated for the full React playground route
• confirmed Playground.tsx no longer statically imports graphiql, @graphiql/create-fetcher, @graphiql/toolkit, or GraphiQL CSS

npm run typecheck still fails on existing unrelated repo issues in publish-externals and Algolia/DocSearch theme types, but it reports no errors from src/components/playground.

I could not run Lighthouse locally in this environment because Chrome/Edge were not available in PATH and npx lighthouse timed out while resolving.