refactor(ui): 调整同伴下贡献值统计卡片

- 删除"已解锁上级"项
- "引荐人数"去掉"人"单位
- "已解锁下级"改为"已解锁下"
- 三个统计项改为单行显示

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

frontend/mining-app/lib/presentation/pages/contribution/contribution_page.dart

@@ -554,33 +554,20 @@ class ContributionPage extends ConsumerWidget {
             style: TextStyle(fontSize: 16, fontWeight: FontWeight.bold, color: AppColors.textPrimaryOf(context)),
           ),
           const SizedBox(height: 16),
-          // 第一行
+          // 统计项
           Row(
             children: [
               _buildTeamStatItem(
                 context,
                 '引荐人数',
                 contribution?.directReferralAdoptedCount.toString(),
-                '人',
+                '',
                 isLoading,
               ),
               const SizedBox(width: 16),
               _buildTeamStatItem(
                 context,
-                '已解锁上级',
+                '已解锁下',
-                '15',
-                '',
-                isLoading,
-              ),
-            ],
-          ),
-          const SizedBox(height: 16),
-          // 第二行
-          Row(
-            children: [
-              _buildTeamStatItem(
-                context,
-                '已解锁下级',
                 contribution?.unlockedLevelDepth.toString(),
                 '',
                 isLoading,