Highlight which prototype page(s) were updated by the AI and let the user jump to them

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

Feedback indicates it'd be beneficial to highlight which page(s) changed in the prototype when it's updated by the AI, and allow the user to jump straight to them.

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

When an existing prototype is updated, the only indication of what's changed (aside from checking manually) is the explanation from the AI. This means that the user needs to carefully check the entire prototype after every update to check what's changed.

### How should the feature be implemented?

This should work by comparing the generated JSON for the current and previous prototype.

This could be added next to (or instead of) the AI explanation or on the Structure tab. It's already possible to jump to a particular page in the live preview in the Structure tab.

The live demo should not jump straight to an updated page automatically, as this would clash with #40.

Some sort of diff view for the JSON could be beneficial, but is not mandatory and should not clutter the UI.

### Additional context (if applicable)

_No response_

Start by tracing the Structure tab’s existing page-navigation behavior and the current and previous generated prototype JSON. Compare those representations to identify changed pages, then verify that selecting a changed page jumps in the live preview without automatic navigation or unnecessary UI clutter.

Written by the indexing model from the issue text.