Add a standalone Wizard playground route

[leoafarias]

Summary

• Add a dedicated / route for the standalone Wizard playground and move the editor to /editor
• Wire the playground to load GOOGLE_AI_API_KEY from the repository .env file via --dart-define-from-file
• Update docs and launch config so the playground can be run directly in macOS with the required config
• Simplify the customization sidebar to show the editor only

Testing

• Added widget coverage for the standalone Wizard route, missing-key state, and editor route migration
• Updated existing editor page coverage to use the new /editor entry point