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>

client/index.html

@@ -78,8 +78,8 @@
               data-help="Show the whole page in the window. Pressed by the host, it does so on every screen. Ctrl (Command on a Mac) with the scroll wheel, or with a pen stroke up or down, zooms about the pointer.">Fit</button>
       <button id="zoom_sheet" type="button"
               data-help="Show the page at the scale a new page has: one A4 sheet filling the window. A shrunk page is then seen at its size on the sheet. Pressed by the host, it does so on every screen.">A4</button>
-      <button id="zoom_widen" type="button" hidden
-              data-help="Make the page wider, for everyone, until it fills the window at this zoom. Nothing on it moves.">Widen</button>
+      <button id="zoom_expand" type="button" hidden
+              data-help="Make the page fill the window, for everyone, at this zoom, in both directions. Nothing on it moves.">Expand</button>
       <button id="zoom_shrink" type="button" hidden
               data-help="Make the page, for everyone, just large enough for what is on it, with the margin around. Nothing on it moves.">Shrink</button>
       <label class="toggle" data-help="Let the page grow when your writing reaches an edge. Off, your writing stops at the edge. Images always get the room they need.">