Say "page", not "sheet", everywhere a user sees it

From the development repository at 854009c.

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

docs/guest.md

@@ -58,9 +58,9 @@ explanation appears beneath it.
 
 ### The page
 
-The page is an A4 sheet lying on its side, shown so that it fits the
+The page is A4 in size, lying on its side, shown so that it fits the
 window.  Write
-anywhere on it.  When your writing comes near an edge, the sheet grows in
+anywhere on it.  When your writing comes near an edge, the page grows in
 that direction, in small steps, so there is always room, and the view
 keeps its scale and extends beyond the window; scroll to reach the rest.
 An image the host puts on the page lies at the top, or below the images
@@ -147,20 +147,19 @@ extends beyond the window and you scroll to the rest.  Three buttons and
 two gestures change the view, and none of them changes the page:
 
 * **Fit** shows the whole page in the window.
-* **A4** shows the page at the scale a new page has, one A4 sheet
+* **A4** shows the page at the scale a new page has, a full A4 page
   filling the window, so that a page shrunk to a small exercise is seen
-  at its size on the sheet rather than enlarged to fill the window.
+  at its own size rather than enlarged to fill the window.
 * 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.
 * **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.
+  directions.  The page grows equally on every side, so nothing on it
+  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
+* **Extend** is a box, ticked to begin with.  Ticked, the page grows
   when your writing comes near an edge, so there is always room.
   Unticked, your writing stops short of the edge; a stroke that leaves
   the page resumes when the pen comes back.  Images always get the room
@@ -222,7 +221,7 @@ doing anything.  Where it is saved depends on how you joined.
 and small pictures of the pages kept in them, wherever they are kept.
 Rest the pointer on a page for a second and its facts appear: the first
 text on it, when it was last changed, who wrote on it, what is on it
-and the size of the sheet; on a notebook, how many pages it keeps and
+and the size of the page; on a notebook, how many pages it keeps and
 from when, when it was last written in, and where it is.
 There you can delete pages and notebooks you no longer want: mark them
 with the × at their left, then press **Delete** twice.  In the browser