fix(leaderboard): 修复加载期空白页,加 loading 图标

- leaderboard.json 未注册 ui-skeleton,骨架屏不渲染致 loading 期空白
- 注册 ui-skeleton,骨架屏恢复 shimmer
- 骨架屏上方加主题色旋转加载图标 + "加载中…"

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

pages/leaderboard/leaderboard.wxml

@@ -12,8 +12,12 @@
     </view>
   </view>
 
-  <!-- Loading skeleton -->
+  <!-- Loading: 旋转加载图标 + 骨架屏 -->
   <view class="leaderboard-skeleton" wx:if="{{loading}}">
+    <view class="lb-loading">
+      <view class="lb-loading__spinner"></view>
+      <text class="lb-loading__text">加载中…</text>
+    </view>
     <ui-skeleton variant="list-row" count="{{5}}" />
   </view>