Add option to manually change `PROMPT_PROCESSING_CHUNK_SIZE` in the GUI

[pawelkiszczak]

The value of this element is hardcoded to 512 which may be fine for some MacBooks, however for advanced users this one should be available to play with since it can influence prefill speed a bit.

Code reference: https://github.com/lmstudio-ai/mlx-engine/blob/57634aff2fe2b763093624d0cc0bd2e259845b9b/mlx_engine/cache_wrapper.py#L15