fix: respect user's DiT model selection in Python spawn fallback

[fuleinist]

When Gradio throws an error and triggers the Python spawn fallback (simple_generate.py), the model was hardcoded to acestep-v15-turbo regardless of the user's selection. This caused issue #77. Changes: simple_generate.py adds --config CLI arg; acestep.ts passes params.ditModel to the Python script. Fixes #77.