feat: added --prompt-only flag to preview final generated prompts without generating images

[dereknex]

Summary

• Added --prompt-only flag to preview final generated prompts without generating images
• Shows enhanced prompts with style modifications before paying for generation
• Displays complete configuration including style, model, size, and warnings
• Perfect for prompt debugging, style comparison, and cost-effective workflow optimization

Usage Examples

# See raw prompt without enhancements
snapai icon --prompt "calculator app"  --prompt-only

# See how minimalism style enhances the prompt
snapai icon --prompt "calculator app" --style minimalism --prompt-only