Expand replaces Widen: fill the window in both directions

From the development repository at 3f1da52.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/quickstart.md

@@ -68,7 +68,7 @@ rather than in the browser's storage.
 ## The screen in one paragraph
 
 The toolbar holds the pen, text, eraser and hide tools, colours and
-line widths, the view commands (Fit, A4, Widen, Shrink, Extend), the
+line widths, the view commands (Fit, A4, Expand, Shrink, Extend), the
 host's page buttons (New, Image, Notebooks), your notebook, Keep, Export
 and Session.  Rest the pointer on any button for a second for its
 explanation.  Each participant writes on a layer of their own that