Summary Plots: Add Appearance option "Mean" for Statistics Curve

Made a predefined Appearance option "Mean" for clean look.
Ensure better appearance of realization curves for all ensemble.

<img width="1473" height="896" alt="Image" src="https://github.com/user-attachments/assets/a782aa04-b02e-441d-a897-a1cb8b719c8e" />

<img width="1461" height="851" alt="Image" src="https://github.com/user-attachments/assets/4640c9ef-7837-424b-a436-ea1b5022ef37" />