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/home/today_task_card.dart

@@ -49,7 +49,15 @@ class TodayTaskCard extends StatelessWidget {
   }
 
   _TodayTask _todayTask() {
-    final lesson = lessonById(state.activeLessonId);
+    var lessonId = state.activeLessonId;
+    if (state.completedLessonIds.contains(lessonId)) {
+      final next = state.nextIncompleteLessonId;
+      if (next != null) {
+        lessonId = next;
+      }
+    }
+    final lesson = lessonById(lessonId);
+
     if (state.hasResumableLessonDialogue) {
       return _TodayTask(
         label: '未完成',
@@ -75,17 +83,39 @@ class TodayTaskCard extends StatelessWidget {
     }
     // A0 课程学完、尚未通过阶段评估时，先巩固再评估；通过后回到下一课。
     if (state.a0LessonsComplete && !state.a0Passed) return _stageTask();
+
+    if (state.allLessonsComplete) {
+      return _TodayTask(
+        label: '已学完',
+        title: '所有课程已完成',
+        note: '你已学完全部课程内容！建议保持日常到期复习或自由情境对话。',
+        action: '去复习',
+        minutes: '10 分钟',
+        onPressed: onStartReview,
+      );
+    }
+
     final segments = lesson.segments.length;
-    final segment = state.activeSegmentIndexFor(lesson.id) + 1;
+    final segmentIndex = state.activeSegmentIndexFor(lesson.id);
+    final segment = segmentIndex + 1;
+    final isSegmentResuming = segments > 1 && segmentIndex > 0;
+
     return _TodayTask(
-      label: '今日新课',
+      label: isSegmentResuming ? '继续课程' : '今日新课',
       title:
           '第 ${lesson.number} 课 · ${lesson.title}'
           '${segments > 1 ? ' · 第 $segment/$segments 段' : ''}',
-      note: '预热词汇 → 听说读写 → 对话 → 独立尝试',
-      action: '开始今天的学习',
+      note: isSegmentResuming
+          ? '第 $segmentIndex 段已学完；继续本课下一段情境训练。'
+          : '预热词汇 → 听说读写 → 对话 → 独立尝试',
+      action: isSegmentResuming ? '继续学习第 $segment 段' : '开始今天的学习',
       minutes: '12 分钟',
-      onPressed: onStartLesson,
+      onPressed: () {
+        if (state.activeLessonId != lesson.id) {
+          state.openLesson(lesson.id);
+        }
+        onStartLesson();
+      },
     );
   }