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/core/courses/course_catalog.dart

@@ -18,6 +18,67 @@ final Map<String, LessonDialogue> segmentDialogues = <String, LessonDialogue>{};
 final Map<String, String> segmentGrammarNotes = <String, String>{};
 final List<DialogueScene> practiceScenes = <DialogueScene>[];
 
+/// All distinct English words introduced in each CEFR level, populated from
+/// the course JSON content packs.
+final Map<String, Set<String>> levelVocabularyWords = <String, Set<String>>{};
+
+const List<String> cefrLevels = ['A0', 'A1', 'A2', 'B1'];
+
+/// Registers words for [level] into the vocabulary index.
+void registerLevelWords(String level, Iterable<String> words) {
+  final set = levelVocabularyWords.putIfAbsent(
+    level.toUpperCase(),
+    () => <String>{},
+  );
+  for (final w in words) {
+    final clean = w.toLowerCase().replaceAll("'", '').trim();
+    if (clean.isNotEmpty) set.add(clean);
+  }
+}
+
+/// Fixed demo names and places accepted across all levels as standard practice slots.
+const Set<String> standardDemoEntities = {
+  'alex',
+  'mia',
+  'shen',
+  'bo',
+  'ava',
+  'tom',
+  'sam',
+  'beijing',
+  'shanghai',
+  'hong',
+  'kong',
+  'china',
+  'london',
+};
+
+/// Returns all English words taught in the course up to [level] (inclusive).
+Set<String> taughtCourseWordsUpToLevel(String level) {
+  final targetIdx = cefrLevels.indexOf(level.toUpperCase());
+  final maxIdx = targetIdx < 0 ? 0 : targetIdx;
+  final result = Set<String>.from(standardDemoEntities);
+  for (var i = 0; i <= maxIdx && i < cefrLevels.length; i++) {
+    final words = levelVocabularyWords[cefrLevels[i]];
+    if (words != null) result.addAll(words);
+  }
+  return result;
+}
+
+/// Returns all English words taught in the course up to [targetItemId]'s level.
+Set<String> taughtCourseWordsUpToItem(String targetItemId) {
+  return taughtCourseWordsUpToLevel(itemLevel(targetItemId));
+}
+
+/// All distinct English words across all loaded course packs.
+Set<String> allLoadedCourseWords() {
+  final result = <String>{};
+  for (final words in levelVocabularyWords.values) {
+    result.addAll(words);
+  }
+  return result;
+}
+
 /// Empties every registry, for `CourseRepository.resetForTest`.
 void clearCourseCatalog() {
   courseLessons.clear();
@@ -28,6 +89,7 @@ void clearCourseCatalog() {
   segmentGrammarNotes.clear();
   practiceScenes.clear();
   assessmentPacks.clear();
+  levelVocabularyWords.clear();
 }
 
 /// Every lesson the app can teach, in learning order: A0 first, then the

kouyu_english/lib/core/courses/course_repository.dart

@@ -120,10 +120,57 @@ class CourseRepository {
     _register(pack);
   }
 
+  void _indexPackVocabulary(CoursePack pack) {
+    final words = <String>{};
+    void addText(String? text) {
+      if (text == null || text.isEmpty) return;
+      final matches = RegExp(r"[A-Za-z]+(?:'[A-Za-z]+)?").allMatches(text);
+      for (final m in matches) {
+        final word = m.group(0)!.replaceAll("'", '').toLowerCase();
+        if (word.isNotEmpty) words.add(word);
+      }
+    }
+
+    for (final item in pack.coreItems) {
+      addText(item.en);
+      addText(item.exampleEn);
+      for (final group in item.match) {
+        for (final term in group) {
+          addText(term);
+        }
+      }
+    }
+    for (final rw in pack.receptiveWords) {
+      addText(rw.en);
+    }
+    for (final seg in pack.segments) {
+      addText(seg.listeningText);
+      addText(seg.speakingText);
+      addText(seg.readingText);
+      addText(seg.writingExample);
+      if (seg.vocabulary != null) {
+        for (final v in seg.vocabulary!) {
+          addText(v.word);
+        }
+      }
+    }
+    for (final scene in pack.scenes) {
+      for (final turn in scene.turns) {
+        addText(turn.ai);
+        addText(turn.model);
+      }
+    }
+    for (final mat in pack.materials) {
+      addText(mat.text);
+    }
+    registerLevelWords(pack.level, words);
+  }
+
   /// Adapts one pack into the shared runtime registries. Explicit rules and
   /// texts in the pack win; anything left out is derived from the core items
   /// the part teaches.
   void _register(CoursePack pack) {
+    _indexPackVocabulary(pack);
     final byId = {for (final item in pack.coreItems) item.id: item};
     final scenesById = {for (final scene in pack.scenes) scene.id: scene};