按 review 意见：/media/ 的路径与扩展名列表提到 shared/ 共用

- server/index.js 开头的路由说明补上 /media/
- AUDIO_EXTS 原先客户端（public/js/media.js）与服务端各一份，只改一边就会悄悄漂移：
  现在两边都从 shared/media.js 取；服务端那份不再 export
- isMediaUrl 只有测试在用，删掉（新增 shared/media.js，测试改从那里取前缀/扩展名）
- 顺手做了 review 里「可选」的那点：有些静态托管对不存在的路径回 200 + index.html，
  这种响应解码会静默失败，所以除了 404，声明了非 audio/* 类型的响应也回退到原地址
  （完全没声明 content-type 的不算错——没有证据表明它不是音频）

public/js/audio.js

@@ -216,6 +216,20 @@ export class SfxLimiter {
 
 // ---- manager -----------------------------------------------------------------------------------------------
 
+/**
+ * Could Web Audio decode this response? A host without the `/media/` route answers 404; some static hosts answer a
+ * missing path with 200 + the SPA's index.html instead, and fetching *that* would fail to decode as silently as a
+ * 404 would — so the fallback looks at the declared type too.
+ *
+ * A response that declares no type at all is not treated as wrong: absence of a header is not evidence of an HTML
+ * page, and fetch stubs / minimal hosts legitimately omit it.
+ * @param {{ ok?: boolean, headers?: { get?: (n: string) => string | null } }} res
+ */
+function isAudioResponse(res) {
+  if (!res || !res.ok) return false;
+  const type = res.headers?.get?.('content-type');
+  return !type || /^\s*audio\//i.test(type);
+}
 export class AudioManager {
   /**
    * @param {{ getManifest?: () => any, win?: any }} [opts]
@@ -387,7 +401,11 @@ export class AudioManager {
         // Extension-less URL first so download managers leave the BGM alone; a host without /media/ still works.
         const media = mediaUrl(url);
         let res = await fetch(media);
-        if (!res.ok && media !== url) res = await fetch(url);
+        if (media !== url && !isAudioResponse(res)) {
+          // Drop the unusable response (404, or a 200 that is really index.html) before trying the original URL.
+          try { await res.body?.cancel?.(); } catch { /* the fallback request matters more than draining this one */ }
+          res = await fetch(url);
+        }
         if (!res.ok) throw new Error(`HTTP ${res.status}`);
         const ab = await res.arrayBuffer();
         return await new Promise((resolve) => {