Rework app-shell layout

assets/tree-sidebar.js

@@ -187,25 +187,20 @@
         });
     }
 
-    // Mobile: a smaller FAB stacked above the menu FAB toggles the rail as an
-    // overlay drawer; selecting any entry closes it (mirrors sidebar-fab.js).
+    // Mobile: the tree FAB sits at the bottom of the stacked FAB group and
+    // opens the rail's content in the full-viewport Overlay (overlay.js). The
+    // rail container always exists when not editing, so — per the layout spec —
+    // the FAB always renders; the overlay auto-closes when a folder/file link
+    // inside it navigates. Hidden on desktop by .fab CSS.
     function setupFab() {
         var fab = document.createElement('button');
         fab.type = 'button';
         fab.className = 'btn btn-fab fab fab-tree';
         fab.title = 'Folder tree';
         fab.setAttribute('aria-label', 'Folder tree');
-        fab.setAttribute('aria-expanded', 'false');
         fab.innerHTML = '<svg viewBox="0 0 16 16" width="1em" height="1em" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linejoin="miter"><path d="M1 6h14v8H1zm0 0V4h5l1 2"/></svg>';
         fab.addEventListener('click', function () {
-            var open = container.classList.toggle('is-open');
-            fab.setAttribute('aria-expanded', open ? 'true' : 'false');
-        });
-        container.addEventListener('click', function (e) {
-            if (e.target.closest('a')) {
-                container.classList.remove('is-open');
-                fab.setAttribute('aria-expanded', 'false');
-            }
+            if (typeof openOverlay === 'function') openOverlay(container);
         });
         document.body.appendChild(fab);
     }