fix: sidebar — x-data vide masquait appState() + header Notion-style + null-guards

- Sidebar avait x-data vide qui créait un scope isolé → vars (workspaceMenuOpen,
  toggleWorkspaceMenu, sidebarCollapsed) introuvables → rien ne fonctionnait
- Suppression x-data → héritage du scope appState() parent
- Header restylé Notion: .sidebar-workspace-header avec border-radius + hover bg
- Mouse hover (mouseenter/mouseleave) active wsHeaderHover → fond plus clair
- Null-guards: folderStack[-1] sécurisé, ctxMenu.node && ctxMenu.node.is_folder

static/css/app.css

@@ -128,8 +128,24 @@ input, select, textarea { font-family: inherit; color: inherit; }
   padding: 0;
 }
 
-/* Workspace header */
+/* Workspace header — Notion style */
+.sidebar-workspace-header {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  padding: 4px 8px;
+  margin: 4px 8px;
+  border-radius: 6px;
+  cursor: pointer;
+  transition: background 150ms;
+  min-height: 32px;
+}
+.sidebar-workspace-header:hover,
+.sidebar-workspace-header.hover {
+  background: var(--bg-hover);
+}
 .sidebar-workspace {
+  /* keep for backward compat */
   display: flex;
   align-items: center;
   justify-content: space-between;