Dato Agent: the Agent tab chat opens too narrow #162

Problem

Opening the Dato Agent tab starts the chat as a very narrow column. The right-hand pane is empty and only says that records will open there.

The inspector always shows that right-hand pane, and the plugin can set its width only once, through preferredWidth. The current calculation keeps all but 420px for the record pane. Because window.outerWidth is wider than the space DatoCMS actually has, the chat is squeezed to almost nothing until someone drags the divider.

Expected

The chat should open at a usable width, with the conversation centered when the pane is wide. The record pane should stay large enough to edit a record, and the empty pane should explain that records appear there and that the divider can be dragged.