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/guest.md

@@ -153,11 +153,11 @@ two gestures change the view, and none of them changes the page:
 * When the host presses **Fit** or **A4**, every participant's view
   follows, each in their own window.  Your own Fit and A4 change only
   your screen.
-* **Widen**, which only the host and co-hosts have, makes the page
-  itself wider, for everyone, until it fills the
-  window at the present zoom.  The sheet grows equally on both sides,
-  so nothing on the page moves and nothing is rescaled; there is simply
-  more room at either side.
+* **Expand**, which only the host and co-hosts have, makes the page
+  itself fill the window, for everyone, at the present zoom, in both
+  directions.  The sheet grows equally on every side, so nothing on the
+  page moves and nothing is rescaled; there is simply more room around
+  it.
 * **Shrink**, also for the host and co-hosts, makes the page, for everyone, just large enough for what is on it,
   with the usual margin around.  Nothing on it moves either.
 * **Extend** is a box, ticked to begin with.  Ticked, the sheet grows