Apple is struggling to attract fresh content for its innovative Vision Pro headset, with just a fraction of the apps available when compared to the number developers created for the iPhone and iPad in their first few months. The lack of a “killer app” to encourage customers to pay upwards of $3,500 for an unproven new product is seen as a problem for Apple, as the Vision Pro goes on sale in Europe on Friday.Apple said recently that there are “more than 2,000” apps available for its “spatial comp...