Admin nav IA: add billing/site/growth/system groups with sub links

public/css/admin-components.css

@@ -169,3 +169,29 @@
 .platform-orders-table .governance-hint{
     line-height:1.4;
 }
+
+/* 总台管理：左侧导航二级分组（最小可用版） */
+.sidebar .nav-group{
+    margin:10px 0;
+}
+
+.sidebar .nav-group-title{
+    cursor:pointer;
+    user-select:none;
+    color:#94a3b8;
+    font-size:12px;
+    text-transform:uppercase;
+    letter-spacing:.08em;
+    font-weight:700;
+    margin:0 0 6px;
+    list-style:none;
+}
+
+.sidebar .nav-group summary::-webkit-details-marker{
+    display:none;
+}
+
+.sidebar .nav-item--sub{
+    display:block;
+    padding-left:12px;
+}