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/lib/features/lesson/lesson_flow.dart

@@ -64,6 +64,7 @@ class _LessonFlowState extends State<LessonFlow> {
   @override
   void initState() {
     super.initState();
+    widget.state.addListener(_onStateChange);
     writingController.text = widget.state.lessonWritingDraft;
     independentController.text = widget.state.independentAttemptDraft;
     previewIndex = widget.state.previewIndex
@@ -73,11 +74,16 @@ class _LessonFlowState extends State<LessonFlow> {
 
   @override
   void dispose() {
+    widget.state.removeListener(_onStateChange);
     writingController.dispose();
     independentController.dispose();
     super.dispose();
   }
 
+  void _onStateChange() {
+    if (mounted) setState(() {});
+  }
+
   @override
   Widget build(BuildContext context) {
     final lesson = lessonById(widget.state.activeLessonId);
@@ -230,19 +236,23 @@ class _LessonFlowState extends State<LessonFlow> {
               widget.state.activeSegmentIndexFor(lesson.id) ==
               lesson.segments.length - 1,
           nextSegmentNumber: widget.state.activeSegmentIndexFor(lesson.id) + 2,
-          onFinish: () {
-            widget.state.finishCurrentLessonSegment();
-            widget.onFinish();
-          },
+          onFinish: _finishOrExit,
         );
     }
     return _LessonScope(
       title:
           '第 ${lesson.number} 课 · ${lesson.title} · 第 ${widget.state.activeSegmentIndexFor(lesson.id) + 1}/${lesson.segments.length} 段',
-      onExit: widget.onFinish,
+      onExit: _finishOrExit,
       child: content,
     );
   }
+
+  void _finishOrExit() {
+    if (widget.state.lessonStep == LessonStep.complete) {
+      widget.state.finishCurrentLessonSegment();
+    }
+    widget.onFinish();
+  }
 }
 
 class _LessonScaffold extends StatelessWidget {