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/media.js

@@ -1,4 +1,4 @@
-// Audio URLs for download managers (IDM / 迅雷 / FDM …) — and why they have no extension.
+// audio URL rewriting for download managers (IDM / 迅雷 / FDM …) — see shared/media.js for the why.
 //
 // The game plays audio with fetch() + Web Audio (`audio.js`): it never uses <audio src>, <a download> or a
 // navigation. Download managers do not care — their browser integration hooks XHR/fetch whose URL ends in a
@@ -11,13 +11,10 @@
 //
 // `server/index.js` resolves /media/… back to the real file under public/assets/audio and still answers with
 // `Content-Type: audio/mpeg` + range support; Web Audio sniffs the container, so the URL is all the same to it.
-// A host that does not implement /media/ keeps working: `audio.js` falls back to the original URL on 404.
-
-/** Prefix of the extension-less audio route served by `server/index.js`. */
-export const MEDIA_PREFIX = '/media/';
-
-/** Extensions `/media/…` may resolve to, in the order the server tries them. */
-export const AUDIO_EXTS = Object.freeze(['.mp3', '.m4a', '.aac', '.ogg', '.oga', '.opus', '.wav']);
+// A host that does not implement /media/ keeps working: `audio.js` falls back to the original URL whenever the
+// /media/ response is unusable (a 404, or a 200 that is not audio at all — some static hosts answer a missing
+// path with the SPA's index.html).
+import { MEDIA_PREFIX, AUDIO_EXTS } from '../../shared/media.js';
 
 const AUDIO_PATH = /^\/assets\/audio\/(.+)$/i;
 
@@ -49,12 +46,3 @@ export function mediaUrl(url, origin = globalThis.location?.origin) {
   if (!rest || segments.some((s) => !s || s === '.' || s === '..' || s.startsWith('.'))) return url;
   return `${MEDIA_PREFIX}${rest}${u.search}`;
 }
-
-/**
- * Is this URL already one of the extension-less media paths?
- * @param {string} url
- * @param {string} [origin]
- */
-export function isMediaUrl(url, origin = globalThis.location?.origin) {
-  return mediaUrl(url, origin) !== url;
-}