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/test/sync_test.dart

@@ -169,6 +169,130 @@ void main() {
     });
   });
 
+  group('SyncMerger restores home page position on a fresh device', () {
+    // Mirrors what the server held for a real account: lesson a0-01 was
+    // finished on one device, then another device pushed its stale position
+    // (active lesson a0-01, review due_at overwritten with the push time).
+    SyncPullResponse stalePull() => const SyncPullResponse(
+      serverTime: '2026-09-16T08:00:00Z',
+      progress: SyncProgressPayload(
+        activeLessonId: 'a0-01',
+        completedLessonIds: ['a0-01'],
+        completedSegmentIds: ['a0-01-a'],
+        updatedAt: '2026-09-16T07:54:27Z',
+      ),
+      masteryUpdates: [
+        SyncMasteryItemPayload(
+          itemId: 'A0-P03',
+          checkpoint: 0,
+          status: 'recall',
+          dueAt: '2026-09-16T07:55:13Z',
+          payload: {
+            'label': 'A0-P03',
+            'evidence': ['exposure', 'assisted', 'independentSuccess'],
+            'firstTaughtAt': '2026-09-16T07:49:45Z',
+          },
+          updatedAt: '2026-09-16T07:54:27Z',
+        ),
+      ],
+    );
+
+    test('advances past lessons completed on another device', () {
+      final state = AppState();
+      state.lessonStep = LessonStep.speaking;
+
+      final changed = SyncMerger.applyPullResponse(state, stalePull());
+
+      expect(changed, isTrue);
+      expect(state.completedLessonIds, contains('a0-01'));
+      expect(state.activeLessonId, 'a0-02');
+      expect(state.activeSegmentIndexFor('a0-01'), 0);
+      expect(state.lessonStep, LessonStep.preview);
+    });
+
+    test('keeps a completed lesson the learner reopened locally', () {
+      final state = AppState();
+      state.completedLessonIds.addAll(['a0-01', 'a0-02']);
+      state.completedLessons = 2;
+      state.activeLessonId = 'a0-01';
+
+      SyncMerger.applyPullResponse(state, stalePull());
+
+      expect(state.activeLessonId, 'a0-01');
+    });
+
+    test('never moves the position backwards', () {
+      final state = AppState();
+      state.completedLessonIds.addAll(['a0-01', 'a0-02']);
+      state.activeLessonId = 'a0-03';
+
+      SyncMerger.applyPullResponse(state, stalePull());
+
+      expect(state.activeLessonId, 'a0-03');
+    });
+
+    test('adopts a further remote lesson and segment position', () {
+      final state = AppState();
+      final pull = SyncPullResponse(
+        serverTime: '2026-09-16T08:00:00Z',
+        progress: SyncProgressPayload(
+          activeLessonId: 'a0-04',
+          completedLessonIds: const ['a0-01', 'a0-02', 'a0-03'],
+          completedSegmentIds: const ['a0-01-a', 'a0-02-a', 'a0-03-a', 'a0-04-a'],
+          updatedAt: '2026-09-16T07:54:27Z',
+        ),
+      );
+
+      SyncMerger.applyPullResponse(state, pull);
+
+      expect(state.activeLessonId, 'a0-04');
+      expect(state.activeSegmentIndexFor('a0-04'), 1);
+    });
+
+    test('rebuilds review cards and repairs never-reviewed due dates', () {
+      final state = AppState();
+
+      SyncMerger.applyPullResponse(state, stalePull());
+
+      final review = state.reviewQueue.singleWhere((r) => r.id == 'A0-P03');
+      expect(
+        review.dueAt.toUtc(),
+        DateTime.parse('2026-09-17T07:49:45Z'),
+      );
+      expect(state.mastery['A0-P03']?.status, MasteryStatus.recall);
+    });
+
+    test('equal checkpoint with more remote evidence updates local status', () {
+      final state = AppState();
+      state.mastery['A0-P03'] = const MasteryItem(
+        id: 'A0-P03',
+        label: 'A0-P03',
+        status: MasteryStatus.newItem,
+        evidence: [EvidenceKind.exposure],
+      );
+
+      SyncMerger.applyPullResponse(state, stalePull());
+
+      expect(state.mastery['A0-P03']?.status, MasteryStatus.recall);
+      expect(state.mastery['A0-P03']?.evidence.length, 3);
+    });
+
+    test('push without a review card derives due date from first teaching', () {
+      final state = AppState();
+      state.mastery['A0-P01'] = MasteryItem(
+        id: 'A0-P01',
+        label: 'A0-P01',
+        status: MasteryStatus.newItem,
+        evidence: const [],
+        firstTaughtAt: DateTime.parse('2026-09-16T07:49:45Z'),
+      );
+
+      final req = SyncMerger.buildPushRequest(state);
+
+      expect(req.masteryUpdates.single.dueAt, '2026-09-17T07:49:45.000Z');
+    });
+  });
+
   group('SyncService HTTP operations', () {
     test('testConnection returns true on 200 health check', () async {
       final mockClient = MockClient((request) async {
@@ -278,7 +402,7 @@ void main() {
 
   group('SyncCoordinator Integration', () {
     test('login, syncNow and logout lifecycle', () async {
-      final mockClient = MockClient((request) async {
+      Future<http.Response> handler(http.Request request) async {
         if (request.url.path == '/api/v1/auth/login') {
           return http.Response(
             jsonEncode({
@@ -325,9 +449,9 @@ void main() {
           );
         }
         return http.Response('Not Found', 404);
-      });
+      }
 
-      final service = SyncService(client: mockClient);
+      final service = SyncService(client: MockClient(handler));
       final coordinator = SyncCoordinator.createForTesting(service: service);
       await coordinator.init();
 
@@ -349,6 +473,29 @@ void main() {
       expect(coordinator.lastSyncTime, isNotNull);
       expect(state.completedLessonIds, contains('a0-01'));
 
+      final pullQueries = <Map<String, String>>[];
+      final relogCoordinator = SyncCoordinator.createForTesting(
+        service: SyncService(
+          client: MockClient((request) async {
+            if (request.url.path == '/api/v1/sync/pull') {
+              pullQueries.add(request.url.queryParameters);
+            }
+            return handler(request);
+          }),
+        ),
+      );
+      await relogCoordinator.init();
+      await relogCoordinator.syncNow(AppState());
+      expect(relogCoordinator.lastSyncTime, isNotNull);
+      await relogCoordinator.login(
+        serverUrl: 'http://127.0.0.1:8080',
+        username: 'tester',
+        password: 'password123',
+      );
+      await relogCoordinator.syncNow(AppState());
+      // A fresh login must pull everything, not only changes since last sync.
+      expect(pullQueries.last.containsKey('since'), isFalse);
+
       await coordinator.logout();
       expect(coordinator.isLoggedIn, isFalse);
       expect(coordinator.config.token, isNull);