Feature/replace playground with graphiql

[RVXD]

Summary

Replaces the archived GraphQL Playground with GraphiQL,
the actively maintained official IDE from the GraphQL Foundation.

Fixes #69 — info popups broken in Chrome 127+ due to removal of the deprecated
DOMNodeRemoved mutation event that Playground relied on.

Fixes #70 — Playground is archived and its bundled JS could not be updated.

What changed

• src/Controller.php — removed GraphQLPlayground.init() and the
  Requirements::customScript() call; returns template variables for GraphiQL instead
• templates/DevTools.ss — rewritten to initialise GraphiQL; multiple schemas
  are handled via a dropdown switcher above the editor
• client/ — replaced bundle.js with locally bundled GraphiQL 3.9.0,
  React 18.3.1, and React DOM 18.3.1 (offline-capable, no build step required)
• README.md — updated to reflect the migration and document how to upgrade
  the bundled files in future

Testing

Open /dev/graphql/ide and verify:

• GraphiQL loads and renders the editor
• Schema introspection completes (docs panel populates)
• Queries execute correctly with the CSRF token
• Multiple schemas show a dropdown switcher (if configured)
• Info popups dismiss correctly in Chrome 127+