refactor: 抽出语音作答与录音回听的公共逻辑

对话页、AI 补练页、课程跟读步骤、独立表达步骤和评估页各自复制了一套
"录音 → AI 转写 → 填入答案"的流程，其中 3 页还复制了"录音回听/播放/删除"
的按钮和方法。现在统一到 lib/widgets/voice_answer.dart：
- VoiceAnswerMixin：startVoiceInput / finishVoiceInput / toggleRecording /
  playRecording / deleteRecording / disposeVoiceAnswer，状态字段名沿用各页原名，
  页面的 build 代码基本不动；
- RecordingControls：录音回听按钮行。

用户可见的变化：
- "未识别到语音"提示统一为「未识别到清晰语音，请再试一次或直接输入文字。」
  （对话页、补练页、独立表达步骤、评估页原本各有不同说法；跟读步骤保留
  原来提示"播放示范音"的文案）。麦克风不可用、开始录音的提示保持各页原样。
- 对话页的录音回听按钮与播放按钮之间增加 8px 间距，与其他页面一致。
- 删除录音时先停止正在播放的录音（原来只有跟读步骤这样做）。
- 离开页面时，如果还在录音或设备识别中，会先停止；不保留录音时一并删除
  这段录音文件。原来课程两个步骤离开时不停止录音，各页离开时也不停止
  "录音回听"的录音。

flutter analyze 无问题，flutter test 142 个测试通过；现有测试未覆盖麦克风流程。

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

kouyu_english/lib/features/assessment/assessment_page.dart

@@ -1,4 +1,3 @@
-import '../../core/ai_service.dart';
 import 'package:flutter/material.dart';
 
 import '../../core/app_state.dart';
@@ -7,6 +6,7 @@ import '../../core/assessment_bank.dart';
 import '../../core/models.dart';
 import '../../core/voice_service.dart';
 import '../../widgets/app_widgets.dart';
+import '../../widgets/voice_answer.dart';
 
 class AssessmentPreparationPage extends StatefulWidget {
   const AssessmentPreparationPage({
@@ -129,16 +129,14 @@ class AssessmentPage extends StatefulWidget {
   State<AssessmentPage> createState() => _AssessmentPageState();
 }
 
-class _AssessmentPageState extends State<AssessmentPage> {
+class _AssessmentPageState extends State<AssessmentPage>
+    with VoiceAnswerMixin<AssessmentPage> {
   final controller = TextEditingController();
   final Map<String, bool> results = {};
   int index = 0;
   bool usedMic = false;
   bool transcriptEdited = false;
   String lastTranscript = '';
-  bool listening = false;
-  bool transcribing = false;
-  bool aiVoiceRecording = false;
   bool audioPlayed = false;
   bool speakingUnavailable = false;
   AssessmentRecord? completedRecord;
@@ -160,10 +158,7 @@ class _AssessmentPageState extends State<AssessmentPage> {
   void dispose() {
     VoiceService.instance.stopSpeaking();
     VoiceService.instance.stopListening();
-    VoiceService.instance.stopRecordingPlayback();
-    if (listening || aiVoiceRecording) {
-      VoiceService.instance.stopRecording();
-    }
+    disposeVoiceAnswer(keepRecording: true);
     controller.dispose();
     super.dispose();
   }
@@ -175,62 +170,31 @@ class _AssessmentPageState extends State<AssessmentPage> {
     if (mounted) setState(() => audioPlayed = true);
   }
 
+  @override
+  AppState get voiceState => widget.state;
+
   Future<void> _mic() async {
     if (aiVoiceRecording) {
-      final path = await VoiceService.instance.stopRecording();
-      if (!mounted) return;
-      setState(() {
-        aiVoiceRecording = false;
-        listening = false;
-        transcribing = true;
-      });
-      if (path != null) {
-        final config = widget.state.aiConfig;
-        final transcribed = await AiService.instance.transcribeAudio(
-          filePath: path,
-          provider: config.provider,
-          endpoint: config.endpoint,
-          model: config.model,
-        );
-        if (mounted) {
-          setState(() {
-            transcribing = false;
-            if (transcribed != null && transcribed.trim().isNotEmpty) {
-              controller.text = transcribed.trim();
-              usedMic = true;
-              lastTranscript = transcribed.trim();
-              transcriptEdited = false;
-              speakingUnavailable = false;
-            }
-          });
-          if (transcribed == null || transcribed.trim().isEmpty) {
-            ScaffoldMessenger.of(context).showSnackBar(
-              const SnackBar(content: Text('未识别到清晰语音，请再试一次。')),
-            );
-          }
-        }
-      } else {
-        if (mounted) setState(() => transcribing = false);
-      }
+      await finishVoiceInput(
+        keepAudio: false,
+        onTranscript: (text) {
+          controller.text = text;
+          usedMic = true;
+          lastTranscript = text;
+          transcriptEdited = false;
+          speakingUnavailable = false;
+        },
+      );
       return;
     }
 
-    VoiceService.instance.stopSpeaking();
-    final recordStarted = await VoiceService.instance.startRecording();
+    final recordStarted = await startVoiceInput(
+      unavailableMessage: '无法访问麦克风，口语可稍后补测。',
+    );
     if (!mounted) return;
-    setState(() {
-      aiVoiceRecording = recordStarted;
-      listening = recordStarted;
-      speakingUnavailable = !recordStarted;
-    });
+    setState(() => speakingUnavailable = !recordStarted);
     if (recordStarted) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('已启动麦克风录音，回答后再次点击，将自动转写为英文。')),
-      );
-    } else {
-      ScaffoldMessenger.of(context).showSnackBar(
-        const SnackBar(content: Text('无法访问麦克风，口语可稍后补测。')),
-      );
+      showVoiceMessage('已启动麦克风录音，回答后再次点击，将自动转写为英文。');
     }
   }