Switch API docs from Swagger UI to Stoplight Elements

Replaces the Swagger UI CDN bundle with @stoplight/elements@9.0.19 web
component. Elements offers better dark mode support and richer Markdown
rendering in endpoint descriptions.

Two Swagger-specific behaviors are not carried over:
- operationsSorter (GET/POST/PUT/PATCH/DELETE order): Elements renders
  operations in spec order, which already follows that convention.
- requestInterceptor that decoded %2F back to / in path parameters:
  worth checking whether Elements encodes these differently during
  interactive testing.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>