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/js/page_model.js

@@ -337,7 +337,8 @@ export function cleanImage(received) {
  * natural size: one way for every image (Andy, 2026-10-08, dropping the
  * shrink-to-fit choice and composition mode, since an image can be moved
  * and scaled by hand).  An image wider than the page widens the sheet
- * equally on both sides; one wider than a page can be is scaled to that.
+ * equally on both sides, and one taller lengthens it downward, so the
+ * whole image has room; one larger than a page can be is scaled to that.
  * @param {Page} page
  * @param {number} naturalWidth   In page units.
  * @param {number} naturalHeight