fix: 登录同步后首页恢复课程位置与复习卡片

- 拉取合并时还原当前课程与分段位置，位置只前进不后退，跳过其他设备已完成的课
- 由云端掌握项重建复习卡片，并纠正旧客户端写坏的首次复习到期时间
- 同 checkpoint 时以证据更多的一方为准；推送缺卡片项时不再用当前时间作到期时间
- 登录/注册及合并逻辑升级后强制全量拉取一次

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

kouyu_english/lib/core/app_state.dart

@@ -1081,6 +1081,73 @@ class AppState extends ChangeNotifier {
     SyncCoordinator.instance.triggerBackgroundSync(this);
   }
 
+  /// Merges lesson progress pulled from the sync server.
+  ///
+  /// Completed lessons/segments are unioned, and the learner's position only
+  /// ever moves forward: a device that has not caught up yet must never drag
+  /// another device back to an earlier lesson or segment.
+  bool mergeSyncedLessonProgress({
+    required Iterable<String> completedLessons,
+    required Iterable<String> completedSegments,
+    required String remoteActiveLessonId,
+  }) {
+    var changed = false;
+    final completedBefore = completedLessonIds.toSet();
+    for (final id in completedLessons) {
+      if (completedLessonIds.add(id)) changed = true;
+    }
+    for (final id in completedSegments) {
+      if (completedSegmentIds.add(id)) changed = true;
+    }
+    if (this.completedLessons != completedLessonIds.length) {
+      this.completedLessons = completedLessonIds.length;
+      changed = true;
+    }
+
+    int lessonIndex(String id) =>
+        a0SeedLessons.indexWhere((lesson) => lesson.id == id);
+
+    var targetIndex = lessonIndex(activeLessonId);
+    final remoteIndex = lessonIndex(remoteActiveLessonId);
+    if (remoteIndex > targetIndex && isLessonUnlocked(remoteActiveLessonId)) {
+      targetIndex = remoteIndex;
+    }
+    // The server's active lesson can itself be stale. Skip lessons that only
+    // became complete through this pull, but leave a lesson alone when the
+    // learner deliberately reopened it locally after finishing it.
+    while (targetIndex >= 0 &&
+        targetIndex < a0SeedLessons.length - 1 &&
+        !completedBefore.contains(a0SeedLessons[targetIndex].id) &&
+        completedLessonIds.contains(a0SeedLessons[targetIndex].id)) {
+      targetIndex++;
+    }
+
+    var positionChanged = false;
+    if (targetIndex >= 0 && a0SeedLessons[targetIndex].id != activeLessonId) {
+      activeLessonId = a0SeedLessons[targetIndex].id;
+      positionChanged = true;
+    }
+
+    for (final lesson in a0SeedLessons) {
+      final segments = lesson.segments;
+      var firstOpen = segments.indexWhere(
+        (segment) => !completedSegmentIds.contains(segment.id),
+      );
+      if (firstOpen < 0) firstOpen = segments.length - 1;
+      if (firstOpen > activeSegmentIndexFor(lesson.id)) {
+        activeSegmentIndexes[lesson.id] = firstOpen;
+        if (lesson.id == activeLessonId) positionChanged = true;
+        changed = true;
+      }
+    }
+
+    if (positionChanged) {
+      _resetLessonFlow();
+      changed = true;
+    }
+    return changed;
+  }
+
   void _resetLessonFlow() {
     lessonStep = LessonStep.preview;
     previewIndex = 0;

kouyu_english/lib/core/sync/sync_coordinator.dart

@@ -14,6 +14,11 @@ import 'sync_service.dart';
 class SyncCoordinator extends ChangeNotifier {
   static const _prefKey = 'sync_config_v1';
 
+  /// 合并逻辑版本。升级后丢弃旧的增量同步时间戳，强制全量拉取一次，
+  /// 让旧版本已经跳过的课程位置和复习卡片能被重新合并。
+  static const _mergeVersion = 2;
+  static const _mergeVersionKey = 'mergeVersion';
+
   static final SyncCoordinator instance = SyncCoordinator._();
   SyncCoordinator._({SyncService? service}) : _service = service ?? SyncService();
 
@@ -49,6 +54,9 @@ class SyncCoordinator extends ChangeNotifier {
       if (raw != null && raw.isNotEmpty) {
         final map = jsonDecode(raw) as Map<String, dynamic>;
         _config = SyncConfig.fromJson(map);
+        if ((map[_mergeVersionKey] as int? ?? 1) < _mergeVersion) {
+          _config = _withoutLastSyncTime(_config);
+        }
       }
     } catch (e) {
       debugPrint('[SyncCoordinator] init error: $e');
@@ -61,12 +69,23 @@ class SyncCoordinator extends ChangeNotifier {
   Future<void> _saveConfig() async {
     try {
       final prefs = await SharedPreferences.getInstance();
-      await prefs.setString(_prefKey, jsonEncode(_config.toJson()));
+      await prefs.setString(
+        _prefKey,
+        jsonEncode({..._config.toJson(), _mergeVersionKey: _mergeVersion}),
+      );
     } catch (e) {
       debugPrint('[SyncCoordinator] save config error: $e');
     }
   }
 
+  static SyncConfig _withoutLastSyncTime(SyncConfig config) => SyncConfig(
+    serverUrl: config.serverUrl,
+    token: config.token,
+    username: config.username,
+    userId: config.userId,
+    autoSyncEnabled: config.autoSyncEnabled,
+  );
+
   /// 更新服务器地址
   Future<void> updateServerUrl(String newUrl) async {
     _config = _config.copyWith(serverUrl: newUrl.trim());
@@ -98,7 +117,8 @@ class SyncCoordinator extends ChangeNotifier {
         username: username,
         password: password,
       );
-      _config = _config.copyWith(
+      // 新登录的账号必须全量拉取，不能沿用之前的增量时间戳
+      _config = _withoutLastSyncTime(_config).copyWith(
         serverUrl: serverUrl.trim(),
         token: auth.token,
         username: auth.username,
@@ -132,7 +152,8 @@ class SyncCoordinator extends ChangeNotifier {
         username: username,
         password: password,
       );
-      _config = _config.copyWith(
+      // 新登录的账号必须全量拉取，不能沿用之前的增量时间戳
+      _config = _withoutLastSyncTime(_config).copyWith(
         serverUrl: serverUrl.trim(),
         token: auth.token,
         username: auth.username,

kouyu_english/lib/core/sync/sync_merger.dart

@@ -1,3 +1,4 @@
+import '../a0_core.dart';
 import '../models.dart';
 import '../app_state.dart';
 import 'sync_models.dart';
@@ -20,8 +21,11 @@ class SyncMerger {
     final masteryUpdates = state.mastery.values.map((m) {
       // 查找对应复习到期时间
       final review = state.reviewQueue.where((r) => r.id == m.id).firstOrNull;
-      final dueAt = review?.dueAt.toUtc().toIso8601String() ??
-          DateTime.now().toUtc().toIso8601String();
+      final dueAt = (review?.dueAt ??
+              m.firstTaughtAt?.add(_firstReviewDelay) ??
+              DateTime.now())
+          .toUtc()
+          .toIso8601String();
       final attempts = review?.attempts ?? 0;
       final successfulReviews = review?.successfulReviews ?? 0;
 
@@ -71,60 +75,60 @@ class SyncMerger {
   static bool applyPullResponse(AppState state, SyncPullResponse response) {
     var changed = false;
 
-    // 1. 合并课程关卡 (Union)
+    // 1. 合并课程关卡 (Union)，学习位置只前进不后退
     if (response.progress != null) {
       final p = response.progress!;
-      for (final id in p.completedLessonIds) {
-        if (!state.completedLessonIds.contains(id)) {
-          state.completedLessonIds.add(id);
-          changed = true;
-        }
-      }
-      for (final sid in p.completedSegmentIds) {
-        if (!state.completedSegmentIds.contains(sid)) {
-          state.completedSegmentIds.add(sid);
-          changed = true;
-        }
-      }
-      if (state.completedLessonIds.length != state.completedLessons) {
-        state.completedLessons = state.completedLessonIds.length;
+      if (state.mergeSyncedLessonProgress(
+        completedLessons: p.completedLessonIds,
+        completedSegments: p.completedSegmentIds,
+        remoteActiveLessonId: p.activeLessonId,
+      )) {
         changed = true;
       }
     }
 
-    // 2. 合并复习掌握项 (Max Checkpoint)
+    // 2. 合并复习掌握项 (Max Checkpoint；同 Checkpoint 时证据更多者更新)
     for (final m in response.masteryUpdates) {
       final local = state.mastery[m.itemId];
+      final status = _parseMasteryStatus(m.status);
+      final evidence = (m.payload['evidence'] as List? ?? [])
+          .whereType<String>()
+          .map(_parseEvidenceKind)
+          .toList();
+      final firstTaughtAt = m.payload['firstTaughtAt'] is String
+          ? DateTime.tryParse(m.payload['firstTaughtAt'] as String)
+          : null;
+      final needsReview = m.payload['needsReview'] as bool?;
       if (local == null) {
         // 本地没有，直接添加
-        final status = _parseMasteryStatus(m.status);
-        final evidence = (m.payload['evidence'] as List? ?? [])
-            .whereType<String>()
-            .map(_parseEvidenceKind)
-            .toList();
-
         state.mastery[m.itemId] = MasteryItem(
           id: m.itemId,
           label: m.payload['label'] as String? ?? m.itemId,
           status: status,
           evidence: evidence,
-          needsReview: m.payload['needsReview'] as bool? ?? false,
+          needsReview: needsReview ?? false,
           checkpoint: m.checkpoint,
-          firstTaughtAt: m.payload['firstTaughtAt'] != null
-              ? DateTime.tryParse(m.payload['firstTaughtAt'] as String)
-              : null,
+          firstTaughtAt: firstTaughtAt,
         );
         changed = true;
-      } else if (m.checkpoint > local.checkpoint) {
-        // 云端 Checkpoint 更高，升级本地状态
-        final status = _parseMasteryStatus(m.status);
+      } else if (m.checkpoint > local.checkpoint ||
+          (m.checkpoint == local.checkpoint &&
+              evidence.length > local.evidence.length)) {
+        // 云端进度更新（证据只追加不删除，条数更多说明学得更多）
         state.mastery[m.itemId] = local.copyWith(
           checkpoint: m.checkpoint,
           status: status,
-          needsReview: m.payload['needsReview'] as bool? ?? local.needsReview,
+          evidence: evidence.length > local.evidence.length ? evidence : null,
+          needsReview: needsReview ?? local.needsReview,
+          firstTaughtAt: local.firstTaughtAt ?? firstTaughtAt,
         );
         changed = true;
+      } else if (local.firstTaughtAt == null && firstTaughtAt != null) {
+        state.mastery[m.itemId] = local.copyWith(firstTaughtAt: firstTaughtAt);
+        changed = true;
       }
+
+      if (_mergeReviewItem(state, m, firstTaughtAt)) changed = true;
     }
 
     // 3. 合并用户偏好设置 (按需合并)
@@ -146,6 +150,53 @@ class SyncMerger {
     return changed;
   }
 
+  /// 与 AppState 引入新课目标时的首次复习间隔保持一致
+  static const _firstReviewDelay = Duration(days: 1);
+
+  /// 首页的到期复习数来自 reviewQueue，因此云端掌握项也要还原为复习卡片
+  static bool _mergeReviewItem(
+    AppState state,
+    SyncMasteryItemPayload m,
+    DateTime? firstTaughtAt,
+  ) {
+    // 只有在课程里正式引入过、或复习过的项目才会有复习卡片
+    if (firstTaughtAt == null && m.attempts == 0) return false;
+    // 从未复习过的卡片到期时间固定为首次学习后一天；旧版本客户端会把
+    // 缺失卡片的 due_at 写成推送时刻，这里据此纠正
+    final dueAt = m.attempts == 0 && firstTaughtAt != null
+        ? firstTaughtAt.add(_firstReviewDelay)
+        : DateTime.tryParse(m.dueAt);
+    if (dueAt == null) return false;
+
+    final index = state.reviewQueue.indexWhere((r) => r.id == m.itemId);
+    if (index < 0) {
+      final template = coreReviewTemplate(m.itemId);
+      state.reviewQueue.add(
+        ReviewItem(
+          id: m.itemId,
+          target: a0CoreItems[m.itemId] ?? m.itemId,
+          prompt: template.prompt,
+          hint: template.hint,
+          dueAt: dueAt.toLocal(),
+          skill: template.skill,
+          attempts: m.attempts,
+          successfulReviews: m.successfulReviews,
+        ),
+      );
+      return true;
+    }
+    final local = state.reviewQueue[index];
+    if (m.attempts > local.attempts) {
+      state.reviewQueue[index] = local.copyWith(
+        dueAt: dueAt.toLocal(),
+        attempts: m.attempts,
+        successfulReviews: m.successfulReviews,
+      );
+      return true;
+    }
+    return false;
+  }
+
   static MasteryStatus _parseMasteryStatus(String str) {
     return MasteryStatus.values.where((e) => e.name == str).firstOrNull ??
         MasteryStatus.newItem;