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/dialogue/dialogue_flow.dart

@@ -682,18 +682,20 @@ class _DialoguePageState extends State<DialoguePage>
       child: SpacedColumn(
         children: [
           Eyebrow('目标：${script.goal}'),
-          Container(
-            constraints: const BoxConstraints(minHeight: 250),
-            child: ListView.separated(
-              shrinkWrap: true,
-              physics: const NeverScrollableScrollPhysics(),
-              itemCount: turns.length,
-              separatorBuilder: (_, _) => const SizedBox(height: 10),
-              itemBuilder: (context, index) => _TurnBubble(
-                turn: turns[index],
-                state: widget.state,
-                showTranslation: _shownTranslations.contains(index),
-                onToggleTranslation: () => _toggleTurnTranslation(index),
+          RepaintBoundary(
+            child: Container(
+              constraints: const BoxConstraints(minHeight: 250),
+              child: ListView.separated(
+                shrinkWrap: true,
+                physics: const NeverScrollableScrollPhysics(),
+                itemCount: turns.length,
+                separatorBuilder: (_, _) => const SizedBox(height: 10),
+                itemBuilder: (context, index) => _TurnBubble(
+                  turn: turns[index],
+                  state: widget.state,
+                  showTranslation: _shownTranslations.contains(index),
+                  onToggleTranslation: () => _toggleTurnTranslation(index),
+                ),
               ),
             ),
           ),
@@ -750,13 +752,22 @@ class _DialoguePageState extends State<DialoguePage>
               ),
             TextField(
               controller: controller,
-              onChanged: (value) => setState(() {
-                if (usedVoice && value != lastTranscript) {
-                  transcriptEdited = true;
+              onChanged: (value) {
+                final needResetVoice =
+                    usedVoice &&
+                    value != lastTranscript &&
+                    !transcriptEdited;
+                final needClearAi = aiCheck != null || aiCheckError != null;
+                if (needResetVoice || needClearAi) {
+                  setState(() {
+                    if (needResetVoice) transcriptEdited = true;
+                    if (needClearAi) {
+                      aiCheck = null;
+                      aiCheckError = null;
+                    }
+                  });
                 }
-                aiCheck = null;
-                aiCheckError = null;
-              }),
+              },
               onSubmitted: (_) => send(),
               decoration: InputDecoration(
                 hintText: '输入你的英文回答',
@@ -785,21 +796,27 @@ class _DialoguePageState extends State<DialoguePage>
                 border: const OutlineInputBorder(),
               ),
             ),
-            OutlinedButton.icon(
-              onPressed:
-                  controller.text.trim().isEmpty ||
-                      checkingWithAi ||
-                      waitingForReply
-                  ? null
-                  : _checkWithAi,
-              icon: checkingWithAi
-                  ? const SizedBox(
-                      width: 16,
-                      height: 16,
-                      child: CircularProgressIndicator(strokeWidth: 2),
-                    )
-                  : const Icon(Icons.spellcheck),
-              label: Text(checkingWithAi ? '正在检查…' : '发送前 AI 检查语法和拼写（可选）'),
+            ValueListenableBuilder<TextEditingValue>(
+              valueListenable: controller,
+              builder: (context, value, _) {
+                final canCheck =
+                    value.text.trim().isNotEmpty &&
+                    !checkingWithAi &&
+                    !waitingForReply;
+                return OutlinedButton.icon(
+                  onPressed: canCheck ? _checkWithAi : null,
+                  icon: checkingWithAi
+                      ? const SizedBox(
+                          width: 16,
+                          height: 16,
+                          child: CircularProgressIndicator(strokeWidth: 2),
+                        )
+                      : const Icon(Icons.spellcheck),
+                  label: Text(
+                    checkingWithAi ? '正在检查…' : '发送前 AI 检查语法和拼写（可选）',
+                  ),
+                );
+              },
             ),
             if (aiCheck != null)
               SectionCard(