fix: support Gradio 6 styling kwargs without breaking Gradio 5

[voidborne-d]

Summary

Make the Gradio demo work across both Gradio 5 and 6 without pinning the dependency.

Changes

• add a small compatibility helper that routes css and theme to gr.Blocks(...) when the installed Gradio version still supports them
• pass those styling kwargs to demo.launch(...) when running on Gradio 6, where they were moved
• add a lightweight regression test covering both API shapes

Testing

• python3 -m unittest tests.test_gradio_compat -v

Fixes #286