feat: 管理后台目录树支持折叠/展开

- 有子节点的目录前显示折叠箭头, 点击折叠/展开子目录
- 折叠状态持久化到 localStorage, 刷新后保留
- 与前台目录树折叠行为一致

web/admin/index.html

@@ -120,6 +120,27 @@
         }
         .tree-item .actions button:hover { background: #e8eaed; }
         .tree-children { padding-left: 20px; }
+        .tree-item .toggle {
+            width: 18px;
+            height: 18px;
+            margin-right: 2px;
+            display: flex;
+            align-items: center;
+            justify-content: center;
+            cursor: pointer;
+            border: none;
+            background: transparent;
+            padding: 0;
+            color: #888;
+            flex-shrink: 0;
+            transition: transform 0.15s ease;
+        }
+        .tree-item .toggle svg { width: 14px; height: 14px; fill: currentColor; }
+        .tree-item .toggle.collapsed { transform: rotate(-90deg); }
+        .tree-item .toggle:hover { color: #1a73e8; }
+        .tree-children { display: block; }
+        .tree-children.hidden { display: none; }
+
         
         /* 右侧内容区 - 左右分栏 */
         .content {
@@ -900,6 +921,15 @@
         const API = '/api';
         const ADMIN_API = '/admin/api';
         let treeData = [];
+        // 折叠的目录 id 集合（持久化到 localStorage，按用户隔离）
+        let collapsedFolders = new Set();
+        try {
+            const saved = JSON.parse(localStorage.getItem('noteManagerCollapsedFolders') || '[]');
+            collapsedFolders = new Set(Array.isArray(saved) ? saved : []);
+        } catch (e) { collapsedFolders = new Set(); }
+        function saveCollapsedFolders() {
+            try { localStorage.setItem('noteManagerCollapsedFolders', JSON.stringify([...collapsedFolders])); } catch (e) {}
+        }
         let currentItem = null;
         let isFolderMode = false;
         let editId = null;                       // 当前打开的笔记 id
@@ -1135,10 +1165,17 @@
                 const children = treeData.filter(c => c.parent_id === item.id);
                 const hasChildren = children.length > 0;
                 const isActive = currentItem && currentItem.id === item.id;
-                
+
                 if (item.is_folder) {
+                    // 目录折叠状态：默认展开；在 collapsedFolders 中则折叠
+                    const isCollapsed = collapsedFolders.has(item.id);
                     return `
                         <div class="tree-item folder ${isActive ? 'active' : ''}" onclick="selectItem(${item.id})">
+                            ${hasChildren ? `
+                            <button class="toggle ${isCollapsed ? 'collapsed' : ''}" title="${isCollapsed ? '展开' : '折叠'}" onclick="event.stopPropagation(); toggleFolder(${item.id})">
+                                <svg viewBox="0 0 24 24"><path d="M8.59 16.59L13.17 12 8.59 7.41 10 6l6 6-6 6z"/></svg>
+                            </button>` : `
+                            <span class="toggle" style="visibility:hidden;"><svg viewBox="0 0 24 24"><path d="M8.59 16.59L13.17 12 8.59 7.41 10 6l6 6-6 6z"/></svg></span>`}
                             <span class="icon">
                                 <svg width="18" height="18" viewBox="0 0 24 24" fill="${isActive ? '#1a73e8' : '#fbbc04'}">
                                     <path d="M10 4H4c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V8c0-1.1-.9-2-2-2h-8l-2-2z"/>
@@ -1150,11 +1187,12 @@
                                 <button onclick="event.stopPropagation(); showNewNoteModal(${item.id})">+笔记</button>
                             </div>
                         </div>
-                        ${hasChildren ? `<div class="tree-children">${renderTreeItems(children)}</div>` : ''}
+                        ${hasChildren ? `<div class="tree-children ${isCollapsed ? 'hidden' : ''}">${renderTreeItems(children)}</div>` : ''}
                     `;
                 } else {
                     return `
                         <div class="tree-item ${isActive ? 'active' : ''}" onclick="selectItem(${item.id})">
+                            <span class="toggle" style="visibility:hidden;"><svg viewBox="0 0 24 24"><path d="M8.59 16.59L13.17 12 8.59 7.41 10 6l6 6-6 6z"/></svg></span>
                             <span class="icon">
                                 <svg width="16" height="16" viewBox="0 0 24 24" fill="#666">
                                     <path d="M19 3H5c-1.1 0-2 .9-2 2v14c0 1.1.9 2 2 2h14c1.1 0 2-.9 2-2V5c0-1.1-.9-2-2-2zm-5 14H7v-2h7v2zm3-4H7v-2h10v2zm0-4H7V7h10v2z"/>
@@ -1167,6 +1205,17 @@
             }).join('');
         }
 
+        // 折叠/展开目录
+        function toggleFolder(id) {
+            if (collapsedFolders.has(id)) {
+                collapsedFolders.delete(id);
+            } else {
+                collapsedFolders.add(id);
+            }
+            saveCollapsedFolders();
+            renderTree();
+        }
+
         // 选择项目
         async function selectItem(id) {
             try {