feat: 优化口语防重复问答、课程推进流转、键盘输入流畅度与竖屏锁定

- AI口语约束：在对话系统提示词与单轮指导中增加防重复问答约束，避免重复寒暄及索取已提供信息，根据历史自然向前推进
- 课程流转：修复多段课程学完后今日任务卡片流转下一课，及退出重进定位逻辑
- 竖屏锁定：在 Flutter、Android 及 iOS 平台配置仅支持竖屏显示
- 对话交互：优化对话界面文本输入重绘与词汇查询，解决键盘输入卡顿
- 测试用例：补充并更新单轮约束、状态流转与返回键退出测试，212项测试全绿

kouyu_english/test/navigation_back_button_test.dart

@@ -8,6 +8,7 @@ import 'package:kouyu_english/features/review/review_page.dart';
 import 'package:kouyu_english/features/shell/learning_shell.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
+import 'package:kouyu_english/core/courses/course_repository.dart';
 import 'support/app_assets.dart';
 
 void main() {
@@ -221,4 +222,83 @@ void main() {
       expect(find.text('四技能评估'), findsOneWidget);
     },
   );
+
+  testWidgets(
+    'LessonFlow on complete step: back button finalizes segment and advances lesson',
+    (tester) async {
+      await tester.runAsync(
+        () => CourseRepository.instance.load(levels: {'A0'}),
+      );
+      final state = AppState()..finishOnboarding();
+      expect(state.activeLessonId, 'a0-01');
+
+      await tester.pumpWidget(MaterialApp(home: LearningShell(state: state)));
+      await tester.pumpAndSettle();
+
+      // Tap '开始今天的学习' to enter LessonFlow
+      await tester.tap(find.text('开始今天的学习'));
+      await tester.pumpAndSettle();
+
+      // Complete all steps of a0-01
+      state.completePreview();
+      state.completeListening(recognized: true);
+      state.completeSpeaking();
+      state.completeReading();
+      state.completeWriting(assisted: false, rawAnswer: 'hello');
+      state.completeLessonDialogue();
+      state.completeIndependentAttempt(assisted: false, rawAnswer: 'hello');
+      expect(state.lessonStep, LessonStep.complete);
+      await tester.pumpAndSettle();
+
+      // We should be in lesson flow complete screen
+      expect(find.text('本段已保存'), findsOneWidget);
+      expect(find.text('回到首页'), findsOneWidget);
+      expect(find.byIcon(Icons.arrow_back), findsOneWidget);
+
+      // Tap the top-left AppBar back button instead of '回到首页'
+      await tester.tap(find.byIcon(Icons.arrow_back));
+      await tester.pumpAndSettle();
+
+      // Should be back on the 学习 tab, and lesson must have advanced to a0-02
+      expect(state.completedLessonIds, contains('a0-01'));
+      expect(state.activeLessonId, 'a0-02');
+      expect(find.textContaining('第 2 课 · '), findsWidgets);
+    },
+  );
+
+  testWidgets(
+    'TodayTaskCard reflects multi-segment progress and completed fallback',
+    (tester) async {
+      await tester.runAsync(
+        () => CourseRepository.instance.load(levels: {'A0'}),
+      );
+      final state = AppState()..finishOnboarding();
+      state.activeLessonId = 'a0-04'; // a0-04 has 3 segments
+
+      await tester.pumpWidget(MaterialApp(home: LearningShell(state: state)));
+      await tester.pumpAndSettle();
+
+      // Segment 1 (initial)
+      expect(find.text('今日新课'), findsOneWidget);
+      expect(find.text('开始今天的学习'), findsOneWidget);
+
+      // Now complete segment 1
+      state.completeSegment('a0-04', 0);
+      await tester.pumpAndSettle();
+
+      // Segment 2 (resuming)
+      expect(find.text('继续课程'), findsOneWidget);
+      expect(find.text('继续学习第 2 段'), findsOneWidget);
+      expect(find.textContaining('第 2/3 段'), findsWidgets);
+
+      // If activeLessonId is marked as completed, card targets next incomplete
+      state.completedLessonIds.add('a0-04');
+      state.notifyListeners();
+      await tester.pumpAndSettle();
+
+      // Should target a0-01 or next incomplete, not show completed lesson
+      expect(find.text('继续课程'), findsNothing);
+      expect(find.textContaining('第 1 课 · '), findsWidgets);
+    },
+  );
 }