feat: add schema-driven Tailcall config builder

[HanTechnology]

/claim #373

Summary

• Add a schema-driven Tailcall config builder at /app/config
• Load the live runtime JSON schema and render the main configuration sections for guided editing
• Support JSON, YAML, and GraphQL output with copy/download actions
• Allow importing an existing JSON config for editing
• Link the builder from /playground

Closes #373

Verification

• npm run build
• npx prettier --check src/components/config-builder/ConfigBuilder.tsx src/pages/app/config.tsx src/components/playground/Playground.tsx
• git diff --check
• Browser: imported {"server":{"hostname":"0.0.0.0","port":8080},"upstream":{"httpCache":42}}, switched to GraphQL, confirmed @server and @upstream output
• Browser: confirmed /playground/ exposes the Config Builder link

Notes

• npm run typecheck -- --pretty false still fails in existing unrelated areas: publish-externals/src/utils/hashnode.ts, src/theme/SearchBar/index.tsx, and src/theme/SearchPage/index.tsx. The modified Playground fetcher typing is clean after this change.