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>

client/index.html

@@ -77,7 +77,7 @@
       <button id="zoom_fit" type="button"
               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>
+              data-help="Show the page at the scale a new page has: a full A4 page filling the window. A shrunk page is then seen at its own size rather than enlarged. Pressed by the host, it does so on every screen.">A4</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