fix: retry TTS_BUSY up to 3 times with backoff

Auto-retries on TTS_BUSY (2s/4s/6s backoff) so pre-warm race at
startup is transparent to the user.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -671,20 +671,27 @@ async function tryVoicevox(text) {
     if (!apiKey) return false;
 
     const ctx = getAudioCtx();
+    for (let attempt = 0; attempt < 4; attempt++) {
+        if (attempt > 0) await new Promise(r => setTimeout(r, 2000 * attempt));
         const res = await fetch(TTS_API, {
             method: 'POST',
             headers: { 'Content-Type': 'application/json', 'Authorization': `Bearer ${apiKey}` },
             body: JSON.stringify({ text: preprocessText(text), speaker: ttsSpeaker }),
         });
-    if (!res.ok) {
-        const msg = res.status === 401 ? 'APIキーが無効です' : res.status === 503 ? 'VOICEVOXが起動していません' : `サーバーエラー (${res.status})`;
+        if (res.status === 503) {
+            const body = await res.json().catch(() => ({}));
+            if (body.error === 'TTS_BUSY' && attempt < 3) continue; // 自動リトライ
+            const msg = body.error === 'TTS_BUSY' ? 'サーバーが混雑しています' : 'VOICEVOXが起動していません';
+            if (currentIdx === 0 && typeof showToast === 'function') showToast('VOICEVOX: ' + msg);
+            return false;
+        }
+        if (!res.ok) {
+            const msg = res.status === 401 ? 'APIキーが無効です' : `サーバーエラー (${res.status})`;
             if (currentIdx === 0 && typeof showToast === 'function') showToast('VOICEVOX: ' + msg);
             return false;
         }
-
         const buf = await ctx.decodeAudioData(await res.arrayBuffer());
         if (!isPlaying) return true;
-
         return new Promise(resolve => {
             currentSource = ctx.createBufferSource();
             currentSource.buffer = buf;
@@ -694,6 +701,8 @@ async function tryVoicevox(text) {
             currentSource.start(0);
             updateVoiceBadge(true);
         });
+    }
+    return false;
 }
 
 function speakBrowser(text, onEnd) {