Missing "line length" setting for note editor

The MacOS and Electron clients have a "line length" (width) setting:

This setting does not exist on the iOS client — let's consider adding it. (It would be especially nice to have for iPads.)

Start by examining how the existing line length setting works in the MacOS and Electron clients, then find the corresponding note editor settings and layout entry points in the iOS client. Done means iOS, especially iPad, offers an equivalent line-length setting that changes the editor width consistently.

Written by the indexing model from the issue text.