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/shell/learning_shell.dart

@@ -71,6 +71,9 @@ class _LearningShellState extends State<LearningShell> {
   void handleBack() {
     switch (route) {
       case _ShellRoute.lesson:
+        if (widget.state.lessonStep == LessonStep.complete) {
+          widget.state.finishCurrentLessonSegment();
+        }
         showTab(tab);
       case _ShellRoute.dialogue:
         if (dialogueInLesson) {
@@ -99,66 +102,6 @@ class _LearningShellState extends State<LearningShell> {
 
   @override
   Widget build(BuildContext context) {
-    Widget body;
-    switch (route) {
-      case _ShellRoute.lesson:
-        body = LessonFlow(
-          // 换课或换段时重建，避免上一段的听力选择和播放状态沿用过来。
-          key: ValueKey(
-            '${widget.state.activeLessonId}-'
-            '${widget.state.activeSegmentIndexFor(widget.state.activeLessonId)}',
-          ),
-          state: widget.state,
-          onOpenDialogue: () => showDialogue(inLesson: true),
-          onFinish: () => showTab(tab),
-        );
-      case _ShellRoute.dialogue:
-        body = DialoguePage(
-          key: ValueKey(dialogueInLesson ? 'lesson' : 'scene-$sceneId'),
-          state: widget.state,
-          isLessonDialogue: dialogueInLesson,
-          sceneId: sceneId,
-          onFinished: (summary) {
-            if (dialogueInLesson) {
-              showLesson();
-            } else if (summary != null) {
-              showSummary(summary);
-            } else {
-              handleBack();
-            }
-          },
-        );
-      case _ShellRoute.summary:
-        body = DialogueSummaryPage(
-          summary: dialogueSummary!,
-          onHome: () => showTab(AppTab.learn),
-          onLesson: showLesson,
-          onRetry: () => showDialogue(scene: sceneId),
-          onBack: () => showTab(tab),
-        );
-      case _ShellRoute.adaptiveLesson:
-        body = AdaptiveLessonPage(
-          state: widget.state,
-          onFinished: () => showTab(AppTab.review),
-        );
-      case _ShellRoute.assessment:
-        body = AssessmentPage(
-          state: widget.state,
-          pack: assessmentPack!,
-          onFinished: () => showTab(AppTab.profile),
-          onStartReplacement: showAssessment,
-        );
-      case _ShellRoute.assessmentPreparation:
-        body = AssessmentPreparationPage(
-          state: widget.state,
-          pack: assessmentPack!,
-          onStart: startAssessment,
-          onBack: () => showTab(AppTab.profile),
-        );
-      case _ShellRoute.tab:
-        body = _tabContent();
-    }
-
     return PopScope(
       canPop: route == _ShellRoute.tab && tab == AppTab.learn,
       onPopInvokedWithResult: (didPop, _) {
@@ -168,8 +111,69 @@ class _LearningShellState extends State<LearningShell> {
       },
       child: AnimatedBuilder(
         animation: widget.state,
-        builder: (context, _) => Scaffold(
-          body: body,
+        builder: (context, _) {
+          final Widget body;
+          switch (route) {
+            case _ShellRoute.lesson:
+              body = LessonFlow(
+                // 换课或换段时重建，避免上一段的听力选择和播放状态沿用过来。
+                key: ValueKey(
+                  '${widget.state.activeLessonId}-'
+                  '${widget.state.activeSegmentIndexFor(widget.state.activeLessonId)}',
+                ),
+                state: widget.state,
+                onOpenDialogue: () => showDialogue(inLesson: true),
+                onFinish: () => showTab(tab),
+              );
+            case _ShellRoute.dialogue:
+              body = DialoguePage(
+                key: ValueKey(dialogueInLesson ? 'lesson' : 'scene-$sceneId'),
+                state: widget.state,
+                isLessonDialogue: dialogueInLesson,
+                sceneId: sceneId,
+                onFinished: (summary) {
+                  if (dialogueInLesson) {
+                    showLesson();
+                  } else if (summary != null) {
+                    showSummary(summary);
+                  } else {
+                    handleBack();
+                  }
+                },
+              );
+            case _ShellRoute.summary:
+              body = DialogueSummaryPage(
+                summary: dialogueSummary!,
+                onHome: () => showTab(AppTab.learn),
+                onLesson: showLesson,
+                onRetry: () => showDialogue(scene: sceneId),
+                onBack: () => showTab(tab),
+              );
+            case _ShellRoute.adaptiveLesson:
+              body = AdaptiveLessonPage(
+                state: widget.state,
+                onFinished: () => showTab(AppTab.review),
+              );
+            case _ShellRoute.assessment:
+              body = AssessmentPage(
+                state: widget.state,
+                pack: assessmentPack!,
+                onFinished: () => showTab(AppTab.profile),
+                onStartReplacement: showAssessment,
+              );
+            case _ShellRoute.assessmentPreparation:
+              body = AssessmentPreparationPage(
+                state: widget.state,
+                pack: assessmentPack!,
+                onStart: startAssessment,
+                onBack: () => showTab(AppTab.profile),
+              );
+            case _ShellRoute.tab:
+              body = _tabContent();
+          }
+
+          return Scaffold(
+            body: body,
           bottomNavigationBar: route == _ShellRoute.tab
               ? NavigationBar(
                   selectedIndex: tab.index,
@@ -201,7 +205,8 @@ class _LearningShellState extends State<LearningShell> {
                   ],
                 )
               : null,
-        ),
+          );
+        },
       ),
     );
   }
@@ -211,7 +216,14 @@ class _LearningShellState extends State<LearningShell> {
       case AppTab.learn:
         return HomePage(
           state: widget.state,
-          onStartLesson: showLesson,
+          onStartLesson: () {
+            if (widget.state.completedLessonIds
+                .contains(widget.state.activeLessonId)) {
+              final next = widget.state.nextIncompleteLessonId;
+              if (next != null) widget.state.openLesson(next);
+            }
+            showLesson();
+          },
           onOpenLesson: (id) {
             widget.state.openLesson(id);
             showLesson();