修复展开文件夹后，顶层文件夹无法新增的问题

app/static/css/style.css

@@ -56,6 +56,20 @@
 #sidebar .nav-link:hover { background: rgba(255,255,255,.08); }
 #sidebar .nav-link.active { background: rgba(255,255,255,.15); color: #fff; }
 
+/* 新建一级文件夹按钮：默认稍微淡化，hover 高亮 */
+#sidebar .folder-add-root {
+  opacity: .75;
+  border-radius: 4px;
+  transition: opacity var(--transition), background var(--transition);
+}
+#sidebar .folder-add-root:hover,
+#sidebar .folder-add-root:focus {
+  opacity: 1;
+  background: rgba(255,255,255,.12);
+}
+/* 折叠态隐藏（避免 60px 宽时挤变形） */
+#sidebar.collapsed .folder-add-root { display: none; }
+
 /* 主内容区 */
 #main-content {
   min-width: 0;

app/templates/base.html

@@ -91,17 +91,18 @@
 
       <!-- 文件夹区块 -->
       <li class="mt-2">
-        <div class="d-flex align-items-center justify-content-between px-1 py-1">
-          <button class="nav-link text-white text-start d-flex align-items-center gap-1 p-0 border-0 bg-transparent"
+        <div class="d-flex align-items-center justify-content-between px-1 py-1 gap-2">
+          <button class="btn btn-sm text-white text-start d-flex align-items-center gap-1 p-0 border-0 bg-transparent flex-grow-1"
                   type="button" data-bs-toggle="collapse" data-bs-target="#folderTree"
                   aria-expanded="true" style="font-size:.9rem">
             <i class="bi bi-folder2 me-1"></i>文件夹
             <i class="bi bi-chevron-down small ms-1" id="folderChevron"></i>
           </button>
-          <button class="btn btn-sm p-0 text-white opacity-75 border-0 bg-transparent"
-                  title="新建文件夹"
-                  data-bs-toggle="modal" data-bs-target="#createFolderModal"
-                  onclick="openCreateFolder(null)">
+          <button class="btn btn-sm py-0 px-1 text-white border-0 bg-transparent flex-shrink-0 folder-add-root"
+                  type="button"
+                  title="新建一级文件夹"
+                  style="font-size:1rem;line-height:1"
+                  onclick="event.stopPropagation();openCreateFolder(null);">
             <i class="bi bi-folder-plus"></i>
           </button>
         </div>