fix: 移除榜单失败toast，避免无数据时误导用户以为云端故障

日榜无数据是正常业务场景（用户当天未训练），弹toast会
让用户误解为系统故障。本地fallback已提供足够的降级体验。

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

pages/leaderboard/leaderboard.js

@@ -66,14 +66,11 @@ Page({
       if (cb) cb()
     }).catch((err) => {
       // Cloud function not deployed or network error — fall back to
-      // local data. Only show a toast if we have nothing to display
+      // local data silently. No toast here: empty day-leaderboard is
-      // yet (avoid confusing toast when a retry already has data).
+      // normal (user didn't train today), and a toast would mislead
+      // them into thinking the cloud is broken.
       console.warn('[leaderboard] cloud function failed, using local fallback:', err)
-      const hadData = this.data.rankedList && this.data.rankedList.length > 0
       this._applyLocal()
-      if (!hadData && (!this.data.rankedList || this.data.rankedList.length === 0)) {
-        wx.showToast({ title: '云端榜单暂不可用', icon: 'none', duration: 2000 })
-      }
       if (cb) cb()
     })
   },