feat(web): 侧边栏一级目录分组可折叠

新增 CollapsibleDirectoryGroup 组件，点击目录标题可展开/折叠子菜单，
默认展开，导航到子菜单时自动展开。侧边栏整体折叠时回落到图标模式。

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

apps/web/src/index.css

@@ -866,6 +866,37 @@ body {
   color: var(--erp-text-tertiary);
 }
 
+/* Sidebar collapsible directory group */
+.erp-sidebar-group-toggle {
+  display: flex;
+  align-items: center;
+  padding: 16px 20px 6px;
+  font-size: 11px;
+  font-weight: 600;
+  text-transform: uppercase;
+  letter-spacing: 0.8px;
+  color: var(--erp-text-tertiary);
+  cursor: pointer;
+  user-select: none;
+}
+
+.erp-sidebar-group-toggle:hover {
+  color: var(--erp-text-secondary);
+}
+
+.erp-sidebar-group-arrow {
+  display: flex;
+  align-items: center;
+  margin-right: 6px;
+  font-size: 10px;
+}
+
+.erp-sidebar-group-label {
+  white-space: nowrap;
+  overflow: hidden;
+  text-overflow: ellipsis;
+}
+
 /* ====================================================================
  * MainLayout — CSS classes replacing inline styles
  * ==================================================================== */