fix frame style panel (#2851)

Opacity slider was squished when frame tool was selected. I think it's
because this PR #2847 got rid of the tlui-style-panel div. Putting it
back seems to fix it.

### Change Type

- [x] `patch` — Bug fix



### Release Notes

- Fixes an issue with the opacity slider getting squished.

---------

Co-authored-by: Steve Ruiz <steveruizok@gmail.com>

packages/tldraw/src/lib/ui/components/StylePanel/DefaultStylePanel.tsx

@@ -33,7 +33,7 @@ export const DefaultStylePanel = memo(function DefaultStylePanel({
 
 	return (
 		<div
-			className={classNames('', { 'tlui-style-panel__wrapper': !isMobile })}
+			className={classNames('tlui-style-panel', { 'tlui-style-panel__wrapper': !isMobile })}
 			data-ismobile={isMobile}
 			onPointerLeave={handlePointerOut}
 		>