fix: 补齐书签 subgroups 渲染并对齐 README

- 修复 subgroups 生成但不展示的问题（模板渲染 + 前端结构导出）
- 更新 README：配置示例/完全替换策略/多层级说明与 helper 文档，减少重复说明

templates/components/group.hbs

@@ -15,6 +15,14 @@
     </div>
     
     <div class="group-content">
+        {{#if subgroups}}
+            <div class="subgroups-container" data-container="subgroups">
+                {{#each subgroups}}
+                    {{> group level=4}}
+                {{/each}}
+            </div>
+        {{/if}}
+
         {{#if sites}}
             <div class="sites-grid" data-container="sites">
                 {{#if sites.length}}
@@ -25,8 +33,12 @@
                     <p class="empty-sites">暂无网站</p>
                 {{/if}}
             </div>
-        {{else}}
-            <p class="empty-content">暂无网站</p>
         {{/if}}
+
+        {{#unless subgroups}}
+        {{#unless sites}}
+            <p class="empty-content">暂无网站</p>
+        {{/unless}}
+        {{/unless}}
     </div>
-</div>
+</div>