BARCELONA—Jonathan Andic had it all. A thriving fashion empire, a beautiful young family and the means to frequent exclusive enclaves in sun-kissed Ibiza and snowy St. Moritz. On Tuesday, he was arrested at his home by Spanish police and taken away in handcuffs to be questioned on suspicion of playing a role in the mysterious death of his billionaire father. Isak Andic, the founder of fashion chain Mango, died in late 2024 after falling more than 300 feet into a gorge while out trekking with his son in the Montserrat mountains near Barcelona. A judge overseeing the investigation into the incident is now accusing…