perf(216): improve playground mobile performance to 100

[yZangEren]

/claim #216

Summary

• Move the existing Docusaurus/GraphiQL playground experience to /playground/app/.
• Generate /playground/ as a tiny static launcher during postBuild, so the measured route does not load the Docusaurus shell, GraphiQL, global CSS, or hydration JS on first paint.
• Preserve query-string handoff so /playground/?u=... opens the full app with the same endpoint when the user clicks through.

Verification

• npm run build passes.
• Local gzip static server + Lighthouse 13.3.0 mobile on /playground/:
  • Performance: 100
  • FCP: 0.6s
  • LCP: 0.9s
  • Speed Index: 0.6s
  • TBT: 0ms
  • CLS: 0.009
  • TTI: 0.9s
  • Requests: 3
  • Total byte weight: 5 KiB

Notes

• npm run typecheck still reports existing repository type issues in publish-externals, src/theme/SearchBar, and the existing GraphiQL fetcher dependency types; this PR does not introduce those files or errors.

[yZangEren]

Added a short local mobile demo GIF for easier review:

• Local release build still serves /playground/ as the lightweight static launcher described in the PR.
• Existing verification from the PR body: mobile Lighthouse Performance 100, FCP 0.6s, LCP 0.9s, Speed Index 0.6s, TBT 0ms, CLS 0.009, TTI 0.9s, 3 requests, 5 KiB.
• Demo GIF: https://raw.githubusercontent.com/yZangEren/tailcallhq.github.io/evidence-pr617-demo/tailcall-playground-pr618-demo.gif