View and modify the selected prompt in the "LLM Setup" dialog window

# Description
In the "LLM Setup" dialog window during the LLM-based unit test generation, there is a button next to the line "Select the prompt." This button is unclickable right now. It would be cool to make it open a textarea field where the prompt may be viewed (and modified):

# Definition of Done
* [ ] Make the decorative button in the "LLM Setup" clickable: open a textarea input where the prompt may be viewed (and modified).

Start by locating the "LLM Setup" dialog and the button next to "Select the prompt" in the Kotlin plugin code. Trace how the dialog currently displays the selected prompt, then verify that the button opens an editable textarea and that changes to the prompt are retained for unit test generation.

Written by the indexing model from the issue text.