Segmented button: selected middle item gets rounded corners instead of rectangular edges

**Description**

In segmented button / tab-like selection widgets, the selected option receives fully rounded corners regardless of its position. Middle items should have rectangular (straight) edges on their left and right sides—only the leftmost and rightmost items in the group should have rounded corners on their outer edges.

**Expected behaviour**

- First item: rounded left corners, straight right edge
- Middle items: straight edges on both sides (even when selected)
- Last item: straight left edge, rounded right corners

**Actual behaviour**

When a middle item is selected, it gets rounded corners on all sides, breaking the continuous shape of the segmented control.

**Environment**

- DMS via CachyOS packages
- Niri 25.11
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