Use Ollama for image generation

Since [image generation can now be used directly in Ollama](https://ollama.com/blog/image-generation), it would be useful to add an Ollama provider option in the Image Generation section.

<img width="1180" height="330" alt="Image" src="https://github.com/user-attachments/assets/f3fa6a6e-03c1-4771-9aa8-68f4b497c385" />