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/host.md

@@ -225,10 +225,10 @@ is the "write your answer here" box of a worksheet: draw it on the scan
 where the answer belongs, and the student types into their own box or
 beside yours.  Only its owner can type in a box, as with any text.
 
-**Widen**, among the view buttons, makes the page wider for everyone,
-until it fills your window at your present zoom: the quickest way to
-more room for writing beside an exercise.  The sheet grows equally on
-both sides, so nothing on the page moves.  The page keeps the new width
+**Expand**, among the view buttons, makes the page fill your window for
+everyone, at your present zoom, in both directions: the quickest way to
+more room for writing around an exercise.  The sheet grows equally on
+every side, so nothing on the page moves.  The page keeps the new size
 in everyone's notebook.  **Shrink** is its opposite: the page becomes
 just large enough for everything on it, images and writing, with the
 margin of 5 mm around, which trims a page down to its exercise before you keep
@@ -266,7 +266,7 @@ notebook in the toolbar first.
 In the **Participants** list of the Session window, each guest's name
 has a box **Co-host** beside it.  Ticking it makes that guest a
 co-host: they may then start new pages, put images on the page and
-arrange them, load pages from their notebook, widen, shrink and hide,
+arrange them, load pages from their notebook, expand, shrink and hide,
 exactly as you can, and the buttons for it appear on their toolbar.
 Unticking it takes it back; otherwise it lasts until the session
 ends.  What stays yours alone: making co-hosts, and bringing everyone's