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/deepseek_thinking_test.dart

@@ -110,7 +110,11 @@ void main() {
     expect(last, startsWith('My name is Alex.'));
     expect(
       last,
-      endsWith('After your line the learner has to: say where they are from.'),
+      contains('After your line the learner has to: say where they are from.'),
+    );
+    expect(
+      last,
+      endsWith('Do not repeat any questions or greetings already asked or answered.'),
     );
     expect(messages![0]['content'], isNot(contains('Ask where the learner')));
   });
@@ -127,6 +131,7 @@ void main() {
     );
     expect(a, b);
     expect(a, contains('CEFR A1'));
+    expect(a, contains('Strictly do not repeat any question'));
     expect(a, endsWith('Taught language: Excuse me.; Can you help me?'));
     expect(
       AiService.dialogueSystemPrompt(level: 'A0', allowedLanguage: const []),