fix(image-creator): fix reference image field name and preview display

- Use `image_url` instead of `data`/`url` in image generation API request
- Fix reference image preview not showing when image is a URL
- Default to ChatboxAI "GPT Image" model instead of restoring last used

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>