Rework app-shell layout

assets/page/sidebar-fab.js

@@ -1,23 +1,20 @@
+// Mobile: the right-rail FAB is stacked ABOVE the tree FAB and opens the right
+// rail (TOC + page widgets) in the full-viewport Overlay (overlay.js). It only
+// appears when the rail actually has content — runs after toc.js has had a
+// chance to populate the rail (script order in page/main.html), so an
+// otherwise-empty rail shows no FAB. Hidden on desktop by .fab CSS.
 document.addEventListener("DOMContentLoaded", function () {
   var aside = document.querySelector("aside.sidebar");
   if (!aside || !aside.children.length) return;
 
   var fab = document.createElement("button");
   fab.type = "button";
-  fab.className = "btn btn-fab fab";
-  fab.title = "Menu";
-  fab.setAttribute("aria-label", "Menu");
-  fab.setAttribute("aria-expanded", "false");
+  fab.className = "btn btn-fab fab fab-rail";
+  fab.title = "Contents";
+  fab.setAttribute("aria-label", "Contents");
   fab.textContent = "≡";
   fab.addEventListener("click", function () {
-    var open = aside.classList.toggle("is-open");
-    fab.setAttribute("aria-expanded", open ? "true" : "false");
-  });
-  aside.addEventListener("click", function (e) {
-    if (e.target.tagName === "A") {
-      aside.classList.remove("is-open");
-      fab.setAttribute("aria-expanded", "false");
-    }
+    if (typeof openOverlay === "function") openOverlay(aside);
   });
   document.body.appendChild(fab);
 });