Improve the waiting experience whilst AI is generating the prototype

### What feature do you want to see added?

Improve the waiting experience to show that progress is being made during prototype generation.

### Why do you want this feature? Does it solve a problem?

When the user creates/updates a prototype using the AI, the button shows `Processing...` but there is no other indication that work is being done. This process can involve a long wait, and the user can be left wondering if it's still happening or if it's got stuck.

### How should the feature be implemented?

_No response_

### Additional context (if applicable)

_No response_

No files, tests, or entry points are named. Start by locating the UI handling prototype creation and updates, then determine how to show ongoing progress beyond the existing `Processing...` label; done means users can tell that generation is still active during a long wait.

Written by the indexing model from the issue text.