feat(leaderboard): 下拉刷新动画图标

下拉刷新时顶部显示主题色旋转 spinner + "刷新中…"指示器,
仅下拉刷新显示(pullRefreshing),后台静默刷新保持无感。

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

pages/leaderboard/leaderboard.wxml

@@ -12,6 +12,12 @@
     </view>
   </view>
 
+  <!-- 刷新中指示器(下拉刷新) -->
+  <view class="refresh-indicator" wx:if="{{pullRefreshing}}">
+    <view class="refresh-spinner"></view>
+    <text class="refresh-text">刷新中…</text>
+  </view>
+
   <!-- Loading: 旋转加载图标 + 骨架屏 -->
   <view class="leaderboard-skeleton" wx:if="{{loading}}">
     <view class="lb-loading">