[Feature Request]: Support Stable Cascade

### Is there an existing issue for this?

- [X] I have searched the existing issues and checked the recent builds/commits

### What would your feature do ?

https://github.com/Stability-AI/StableCascade

### Proposed workflow

Stable cascade should be a selectable model option

### Additional information

Probably need separate slider for steps in "Stage C" and "Stage A&B"

Start by reading the Stable Cascade project linked in the issue and tracing how selectable models and generation-step controls are represented in this repository. Done means Stable Cascade is available as a selectable model and separate controls exist for Stage C and Stage A&B steps, but the issue names no files or tests to run.

Written by the indexing model from the issue text.