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/kokkoja.css

@@ -260,6 +260,11 @@ h2 {
   flex: 1;
   min-width: 0;
   overflow: auto;
+  /* Centre the page in the desk both ways, so that growing the sheet
+     ("Expand") equally on every side leaves what is on it where it is
+     seen.  margin:auto on an overflowing flex child computes to 0, so a
+     page larger than the desk still scrolls to every edge. */
+  display: flex;
   padding: 0.5rem;
   background: var(--desk);
 }
@@ -287,7 +292,8 @@ h2 {
 
 .page {
   position: relative;
-  margin: 0 auto;
+  flex: none;
+  margin: auto;
   background: white;
   box-shadow: 0 1px 4px rgb(0 0 0 / 0.25);
   touch-action: none;