Rendering updates to tab groups in pane view

[johnturcoo]

Description

Two small UI fixes for tab groups in the vertical tabs panel's Panes view, so a group reads as a first-class list item — visually consistent with regular (ungrouped) tab containers:

• Group containers now render with flat corners instead of a rounded card.
• Group containers now get the same divider lines that separate regular tabs (a top divider always, plus a bottom divider when the group is the last item in the list).

How it works

Basic rendering changes to tab groups when in pane view. Prevents rounded corners and adds a divider above like regular tabs do in pane view (also adds separator below when its the last item).

Linked Issue

https://linear.app/warpdotdev/issue/APP-4781/vtab-group-small-ui-fixes

Testing

• I have manually tested my changes locally with ./script/run

Screenshots / Videos

Demo before

Demo after