Add Image Generation support (OpenAI: DALL·E, GPT-Image)

[ibelick]

We want to add support for image generation in Zola using OpenAI models, starting with: gpt-image-1, dall-e-3, dall-e-2,

Reference for integration: https://ai-sdk.dev/docs/ai-sdk-core/image-generation

Tasks:

• Add support for image generation prompts
• Render image outputs in the chat UI
• Cache/store generated images in Supabase

[rabden]

why not use gemini 2.0 flash image generation model