AIStudio-v3: Center Stage header back button labeling mismatch

The leftmost button in the Center Stage header should display “Back” or “Prev” when previous experiences exist and should say Chat only when its the only experienced open or its the first one that was opened. Update label dynamically based on navigation state.
https://jam.dev/c/ba26b2a5-fc87-4395-b2d1-cb22155fb72a