Make spacing between browser preview and tab area consistent

### Description

There is slightly uneven spacing between the browser preview container and the tab area in the Agents window. This makes the separation between the two areas look visually inconsistent.

### Steps to reproduce

1. Open the Agents window.
2. Open or view a browser preview.
3. Compare the spacing around the browser preview container with the adjacent tab area.

### Expected behavior

The spacing between the browser preview and tab area is even. Alternatively, the extra container treatment could be removed and replaced with a single dividing line across the top of the browser preview.

### Actual behavior

The gap between the browser preview container and the tab area appears slightly uneven.