fix: prefer browser speech synthesis over Google TTS

Use local zh-CN speech synthesis first for stable offline playback; fall back to Google TTS only when synthesis is unavailable or fails.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: ALiangPang

js/app.js

@@ -60,30 +60,39 @@
     throw new Error('Google TTS failed');
   }
 
-  async function speakGoogle(text, rate = DEFAULT_SPEECH_RATE) {
-    try {
-      await playGoogleTts(text, rate);
-    } catch {
-      speakSynthFallback(text, rate);
-    }
+  function speakWithSynth(text, rate = DEFAULT_SPEECH_RATE) {
+    return new Promise((resolve, reject) => {
+      if (!('speechSynthesis' in window)) {
+        reject(new Error('No speech synthesis'));
+        return;
+      }
+
+      stopSpeechAudio();
+      speechSynthesis.cancel();
+
+      const utterance = new SpeechSynthesisUtterance(text);
+      utterance.lang = 'zh-CN';
+      utterance.rate = rate;
+      const voice = getChineseVoice();
+      if (voice) utterance.voice = voice;
+      utterance.onend = () => resolve();
+      utterance.onerror = () => reject(new Error('Speech synthesis failed'));
+      speechSynthesis.speak(utterance);
+    });
   }
 
-  function speakSynthFallback(text, rate) {
-    if (!('speechSynthesis' in window)) {
-      alert('Không phát được âm thanh. Kiểm tra mạng và thử lại.');
-      return;
+  async function speak(text, rate = DEFAULT_SPEECH_RATE) {
+    try {
+      await speakWithSynth(text, rate);
+    } catch {
+      try {
+        await playGoogleTts(text, rate);
+      } catch {
+        alert('Không phát được âm thanh. Kiểm tra mạng và thử lại.');
+      }
     }
-
-    speechSynthesis.cancel();
-    const utterance = new SpeechSynthesisUtterance(text);
-    utterance.lang = 'zh-CN';
-    utterance.rate = rate;
-    chineseVoice = getChineseVoice();
-    if (chineseVoice) utterance.voice = chineseVoice;
-    speechSynthesis.speak(utterance);
   }
 
-  // --- Speech ---
   function initSpeech() {
     if (!('speechSynthesis' in window)) return;
 
@@ -108,10 +117,6 @@
     );
   }
 
-  function speak(text, rate = DEFAULT_SPEECH_RATE) {
-    speakGoogle(text, rate);
-  }
-
   function flashAudioBtn(btn) {
     if (!btn) return;
     btn.classList.add('playing');