[Vertical Tabs] Fill extra width with pinned tabs in vertical tabs

<img width="341" height="103" alt="Image" src="https://github.com/user-attachments/assets/482f570b-c0af-48c8-8e8b-bf0835ee56f5" />

<img width="318" height="131" alt="Image" src="https://github.com/user-attachments/assets/79f3ab37-41be-4400-bbf7-a66612e823a6" />

Currently, our pinned tabs left extra space next to pinned tabs. We should fill the area