A disabled button is dimmed

From the development repository at 74b5119.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client/kokkoja.css

@@ -36,6 +36,12 @@ button {
 }
 
 button:hover { border-color: var(--muted); }
+/* A button that cannot be used now (Undo with nothing to undo) is dimmed,
+   as every toolkit does; the browser's own grey for it was overridden
+   by the colour every button inherits, so the state had been invisible
+   (Andy, 2026-10-11).  The label keeps its width: nothing moves. */
+button:disabled { color: var(--muted); cursor: default; }
+button:disabled:hover { border-color: var(--line); }
 
 button[aria-pressed="true"] {
   border-color: var(--accent);