fix(sync): 数据同步合并化 + 排行榜缓存与游标分页

- storage: 新增 mergeRecords,云端记录与本地按 id 去重合并,restore
  不再覆盖,解决训练中冷启动丢记录与多设备记录丢失
- storage: 新增 recomputeStreak,restore 后基于合并记录重算 streak,
  不再信任云端 streak 值
- storage: 新增 _local_changed_at 标记,settings/profile/customPlans
  在本地有未同步改动时保留本地,避免被云端旧值覆盖
- app: _restoreFromCloud 改合并 + 时间戳保护 + streak 重算;Case 2
  合并后回推让两端收敛
- leaderboard: 模块级 60s 缓存 + 游标分页(_id > lastId)替代 skip,
  全表扫描从每次打开降到 60s 内一次

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app.js

@@ -67,14 +67,17 @@ App({
 
         if (!hasLocal && hasCloud) {
           // Case 1: Fresh install on a new device — restore from cloud
-          this._restoreFromCloud(cloudData)
+          this._restoreFromCloud(cloudData, cloudTs)
           wx.setStorageSync('_cloud_sync_at', cloudTs)
         } else if (hasLocal && hasCloud) {
           // Case 2: Both sides have data — compare timestamps
           if (cloudTs > localSyncAt) {
-            // Cloud is newer (another device synced after our last push)
-            this._restoreFromCloud(cloudData)
+            // Cloud is newer (another device synced after our last push).
+            // Merge cloud into local, then push the merged result back so
+            // both sides converge (records union + local's unsynced edits).
+            this._restoreFromCloud(cloudData, cloudTs)
             wx.setStorageSync('_cloud_sync_at', cloudTs)
+            cloud.pushAll()
           } else {
             // Local is at least as current — push
             cloud.pushAll()
@@ -90,35 +93,47 @@ App({
     }
   },
 
-  _restoreFromCloud(cloudData) {
+  _restoreFromCloud(cloudData, cloudTs) {
     if (!cloudData) return
     try {
-      if (cloudData.records && Object.keys(cloudData.records).length > 0) {
-        // Guard: only restore if at least one month has actual records.
-        // Pre-6d19928 duplicates could leave docs with month keys but
-        // empty arrays, which would wipe local data on restore.
-        const hasAny = Object.values(cloudData.records).some(
+      // Records: MERGE instead of overwrite. A training saved during the
+      // async pullAll (or edits from another device) would otherwise be
+      // wiped by the cloud's older snapshot.
+      if (cloudData.records) {
+        const merged = storage.mergeRecords(cloudData.records)
+        const hasAny = Object.values(merged).some(
           arr => Array.isArray(arr) && arr.length > 0
         )
         if (hasAny) {
-          wx.setStorageSync('training_records', cloudData.records)
+          wx.setStorageSync('training_records', merged)
         }
       }
-      if (cloudData.settings) {
-        wx.setStorageSync('user_settings', cloudData.settings)
-      }
-      if (cloudData.streak) {
-        wx.setStorageSync('current_streak', cloudData.streak)
-      }
-      if (cloudData.customPlans && typeof cloudData.customPlans === 'object') {
-        wx.setStorageSync('custom_plans', cloudData.customPlans)
-      }
-      if (cloudData.profile && typeof cloudData.profile === 'object') {
-        wx.setStorageSync('user_profile', cloudData.profile)
-      }
-      if (cloudData.themeId) {
-        themeMod.setTheme(cloudData.themeId)
+
+      // Single-value fields: only adopt the cloud version if the local
+      // side hasn't changed since the cloud snapshot. Otherwise we'd
+      // clobber unsynced local edits (e.g. a setting flipped mid-sync).
+      const localChangedAt = wx.getStorageSync('_local_changed_at') || 0
+      const keepLocal = cloudTs > 0 && localChangedAt > cloudTs
+      if (keepLocal) {
+        console.log('[cloud] restore: keeping local single-value fields (local changed after cloud snapshot)')
+      } else {
+        if (cloudData.settings) {
+          wx.setStorageSync('user_settings', cloudData.settings)
+        }
+        if (cloudData.customPlans && typeof cloudData.customPlans === 'object') {
+          wx.setStorageSync('custom_plans', cloudData.customPlans)
+        }
+        if (cloudData.profile && typeof cloudData.profile === 'object') {
+          wx.setStorageSync('user_profile', cloudData.profile)
+        }
+        if (cloudData.themeId) {
+          themeMod.setTheme(cloudData.themeId)
+        }
       }
+
+      // Streak: never trust the cloud value - recompute from the merged
+      // records so it reflects actual history on this device.
+      storage.recomputeStreak()
     } catch (e) {
       console.error('Cloud restore failed:', e)
     }