Dart Simple Chat example should either handle or not allow interruption.

Simple Chat misses clear handling of concurrency: what happens when user clicks a genui UI button in the middle of the model's response? Right now conversation is messed up in this case.

This question should be handled clearly for both **code reader (AI and human) and end user**.

One of options to handle it is:
1. Disable all gen UI while model is thinking
2. Add button to interrupt thinking to enable UI back (or to enable just user chat box)

More advanced option:
1. Create button to pause/resume model - stop processing responses in the middle, maybe storing thinking responses in history. After pausing user will be able to either resume processing or send new message.
2. When user clicks a gen UI button, handle it as pause + send another message.
3. Make user chat box always enabled and using the same mechanism.