feat: add interactive terminal UI (ai -i)

1. ai-cli only had one-shot commands, so trying prompts, models and settings meant retyping long command lines.
2. ai -i opens an interactive terminal UI where you type the prompt and pick everything else, with thumbnails, a click-to-open slider and @last chaining.
3. Related: #113 lets Gemini image models use the count setting.

Demo

A text reply, then two images with settings picked by Tab, and a click on a thumbnail that opens it full screen (played at 2x).

https://github.com/user-attachments/assets/2bd3f749-e5a3-49b0-9857-c5daa7d2e715