fix(admin-web): 引荐关系树总部节点始终显示在顶端

- 总部节点始终显示在引荐人链最顶端
- 总部节点不可点击（cursor: default）
- 连接符方向改为向下（↓）表示引荐关系

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/admin-web/src/app/(dashboard)/users/[id]/page.tsx

@@ -413,7 +413,13 @@ export default function UserDetailPage() {
                   {/* 向上的引荐人链 */}
                   <div className={styles.referralTree__ancestors}>
                     <div className={styles.referralTree__label}>引荐人链 (向上)</div>
-                    {referralTree.ancestors.length > 0 ? (
+                    {/* 总部节点（始终显示在最顶端） */}
+                    <div className={styles.referralTree__headquarters}>
+                      <span className={styles.referralTree__headquartersLabel}>总部</span>
+                    </div>
+                    <div className={styles.referralTree__connector}>↓</div>
+                    {/* 祖先节点列表 */}
+                    {referralTree.ancestors.length > 0 && (
                       <>
                         <div className={styles.referralTree__nodeList}>
                           {referralTree.ancestors.map((ancestor, index) => (
@@ -431,20 +437,12 @@ export default function UserDetailPage() {
                                 </span>
                               </button>
                               {index < referralTree.ancestors.length - 1 && (
-                                <div className={styles.referralTree__connector}>↑</div>
+                                <div className={styles.referralTree__connector}>↓</div>
                               )}
                             </div>
                           ))}
                         </div>
-                        <div className={styles.referralTree__connector}>↑</div>
+                        <div className={styles.referralTree__connector}>↓</div>
-                      </>
-                    ) : (
-                      <>
-                        <div className={styles.referralTree__headquarters}>
-                          <span className={styles.referralTree__headquartersLabel}>总部</span>
-                          <span className={styles.referralTree__headquartersDesc}>顶级用户，无上级引荐人</span>
-                        </div>
-                        <div className={styles.referralTree__connector}>↑</div>
                       </>
                     )}
                   </div>

frontend/admin-web/src/app/(dashboard)/users/[id]/user-detail.module.scss

@@ -482,27 +482,21 @@
 
 // 总部节点样式
 .referralTree__headquarters {
-  @include flex-column;
+  @include flex-center;
-  align-items: center;
+  padding: $spacing-sm $spacing-lg;
-  padding: $spacing-base $spacing-xl;
   background: linear-gradient(135deg, rgba($primary-color, 0.1) 0%, rgba($success-color, 0.1) 100%);
   border: 2px solid $primary-color;
   border-radius: $border-radius-base;
-  min-width: 140px;
+  min-width: 100px;
+  cursor: default;
 }
 
 .referralTree__headquartersLabel {
-  font-size: $font-size-lg;
+  font-size: $font-size-base;
   font-weight: $font-weight-bold;
   color: $primary-color;
 }
 
-.referralTree__headquartersDesc {
-  font-size: $font-size-xs;
-  color: $text-secondary;
-  margin-top: $spacing-xs;
-}
-
 // ============================================================================
 // 认种信息 Tab
 // ============================================================================