Add data breakpoint to layout css (#2076)

This PR adds `data-breakpoint` to the `tlui-layout`.

### Change Type

- [x] `minor` — New feature

packages/tldraw/src/lib/ui/TldrawUi.tsx

@@ -136,6 +136,7 @@ const TldrawUiContent = React.memo(function TldrawUI({
 				className={classNames('tlui-layout', {
 					'tlui-layout__mobile': breakpoint < 5,
 				})}
+				data-breakpoint={breakpoint}
 			>
 				{isFocusMode ? (
 					<div className="tlui-layout__top">