Productionize 'Side-by-side browser and editor'

* [ ] What is the expected behaviour of 'Preview' in the menu if the Card Browser is split? 
  * https://discord.com/channels/368267295601983490/701922522836369498/1464554877463171092
  * > I forgot about [a user wanting to preview all cards]. It’s a bit confusing, and we can’t really support both at the same time, we’ll have to choose one behavior