Add model selection options for the Expand feature similar to what's available in Write.

Why it matters:

Consistency: Keep the same AI model throughout your entire writing process Cost control: Choose more affordable models when needed Style matching: Ensure expansions match the style/tone of your original writing Flexibility: Different projects might benefit from different models

Potential Implementation: Simple dropdown menu next to the Expand button allowing users to select from Claude Sonnet, GPT -4o or other available models.