fix: 缓存击穿/play竞态/onUnload泄漏/清除数据UI/统计NaN

- leaderboard 云函数: _fetchLatestByOpenid 加 in-flight promise,并发请求复用同一次扫描,防缓存击穿
- voice: play 加序列号,丢弃过期播放,避免旧语音截断新语音
- timer: onUnload 补取消 _completionCountUp,系统返回手势不再泄漏动画定时器
- settings: 清除数据 setData 补 voiceGender/vibrateIntensity 重置,避免 UI 与存储不一致
- storage: getTotalStats 对 r.duration 防御(Number||0),避免脏数据产生 NaN 传染

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

pages/settings/settings.js

@@ -371,6 +371,8 @@ Page({
       currentPlanId: 'beginner',
       voiceGuide: true,
       vibrate: true,
+      voiceGender: 'female',
+      vibrateIntensity: 'heavy',
       currentThemeId: 'orange',
       nickName: '',
       avatarUrl: ''