[fix] fit to content shown on groups (#2946)

This PR fixes a bug where "fit to content" would be shown on groups.

### Change Type

- [x] `patch` — Bug fix

### Release Notes

- Fix bug where "fit frame to content" would be shown when a group is
selected.

packages/tldraw/src/lib/ui/components/menu-items.tsx

@@ -87,7 +87,10 @@ export function FitFrameToContentMenuItem() {
 		() => {
 			const onlySelectedShape = editor.getOnlySelectedShape()
 			if (!onlySelectedShape) return false
-			return editor.getSortedChildIdsForParent(onlySelectedShape).length > 0
+			return (
+				editor.isShapeOfType<TLFrameShape>(onlySelectedShape, 'frame') &&
+				editor.getSortedChildIdsForParent(onlySelectedShape).length > 0
+			)
 		},
 		[editor]
 	)