fix(web-integration): improve static report playground execution

[EAGzzyCSL]

Summary

• reuse the serialized UI context in StaticPageAgent so static reports avoid redundant browser-side screenshot decoding and resizing
• preserve serialized screenshot items and invalidate the cached UI context when static context changes
• enable CORS for the Web Playground platform and allow the opaque Origin: null used by file:// reports
• keep the default CORS policy limited to local browser origins and file reports, while allowing callers to disable it explicitly

Validation

• pnpm run lint
• pnpm exec nx test @midscene/playground -- launcher.test.ts
• pnpm exec nx test @midscene/web -- platform.test.ts
• pnpm exec nx test @midscene/web
• pnpm exec nx build @midscene/playground
• pnpm exec nx build @midscene/web
• pnpm exec nx build @midscene/report