AI-generated template candidates for style selection

Summary

Banana Slides already supports uploading a template image and using template_style. I’d like to add a lightweight pre-step that generates 5 AI template candidate images from a style prompt, lets the user pick one, and then reuses the chosen image through the existing template upload flow.

Why

Many users don’t have a reference/template image ready. A small candidate-picking step would make it easier to bootstrap the existing workflow without adding a separate generation path.

Proposed MVP

• User enters a style prompt
• Backend returns 5 template candidates
• User selects one candidate
• The selected image is uploaded through the existing project template endpoint
• Support a mock/stub mode first so the flow can be tested without real image-generation credentials

Non-goals for MVP

• persistent candidate history
• user template library integration
• candidate editing/re-ranking
• replacing the current template upload workflow

If this sounds reasonable, I can follow up with a small PR that keeps the change minimal and compatible with the current project flow.