跟读录音优先选 mp4 格式(Safari/Chrome 通用),解决家长端 iOS 听不到

- MediaRecorder 优先 audio/mp4,回退 webm;iOS Safari 可回放 mp4
- 清理历史 webm 录音(无法在 iOS 播放)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/words/word-day.tsx

@@ -29,7 +29,12 @@ function RecordBtn({ day, word }: { day: number; word: string }) {
   const start = async () => {
     try {
       const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
-      const mr = new MediaRecorder(stream);
+      // 优先选 Safari/Chrome 都能播放的 mp4,家长端 iOS 才能回放
+      const mime =
+        ["audio/mp4", "audio/mp4;codecs=mp4a.40.2", "audio/webm;codecs=opus", "audio/webm"].find(
+          (t) => typeof MediaRecorder !== "undefined" && MediaRecorder.isTypeSupported(t),
+        ) || "";
+      const mr = new MediaRecorder(stream, mime ? { mimeType: mime } : undefined);
       const chunks: Blob[] = [];
       mr.ondataavailable = (e) => chunks.push(e.data);
       mr.onstop = async () => {