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/app_state_test.dart

@@ -1462,4 +1462,22 @@ void main() {
       }
     }
   });
+
+  test('finishCurrentLessonSegment advances activeLessonId to next lesson', () {
+    final state = AppState();
+    expect(state.activeLessonId, '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.lessonCanComplete, isTrue);
+
+    state.finishCurrentLessonSegment();
+
+    expect(state.completedLessonIds, contains('a0-01'));
+    expect(state.activeLessonId, 'a0-02');
+  });
 }