fix(mining-app): hardcode team bonus tiers display to 15

- Profile page: 团队上级 shows '15' instead of actual unlockedBonusTiers
- Contribution page: 已解锁上级 shows '15级' instead of actual value

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

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

@@ -527,7 +527,7 @@ class ContributionPage extends ConsumerWidget {
               const SizedBox(width: 16),
               _buildTeamStatItem(
                 '已解锁上级',
-                contribution?.unlockedBonusTiers.toString(),
+                '15',
                 '级',
                 isLoading,
               ),

frontend/mining-app/lib/presentation/pages/profile/profile_page.dart

@@ -265,7 +265,7 @@ class ProfilePage extends ConsumerWidget {
           _buildDivider(),
           _buildStatItem(
             '团队上级',
-            stats?.unlockedBonusTiers.toString() ?? '0',
+            '15',
             isLoading,
           ),
         ],