fix: audioSession playback type, exponential backoff, pageshow recovery

- Set navigator.audioSession.type = 'playback' on first play — tells Android
  OS this is a music app, keeping background network alive through Doze mode
- Replace flat 2s error retry with exponential backoff (1s/2s/4s/8s)
- Add pageshow event alongside visibilitychange for Samsung/iOS PWA reliability
- Attempt audio resume on foreground if OS killed playback while backgrounded
- Switch preload from hidden audio element to link rel=prefetch (lighter,
  avoids triggering two-audio-stream detection on Android)

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

src/player.js

@@ -5,10 +5,10 @@ import { api, streamUrl, coverUrl, downloadUrl } from './api.js';
 import { loadRecommendations } from './views.js';
 import { updateLyricsHighlight, onNpSongChange } from './nowplaying.js';
 
-// ─── PRELOAD (keeps network alive through Doze mode gap between tracks) ───
-let _preAudio = null;
-let _preUrl   = '';
-let _preId    = '';
+// ─── PRELOAD (resource hint keeps connection open through Doze gap) ───
+let _preUrl  = '';
+let _preId   = '';
+let _preLink = null;
 
 function _nextIdx() {
   if(S.repeat === 'one') return S.qIdx;
@@ -25,14 +25,17 @@ function _preloadNext() {
   if(!next || _preId === next.id) return;
   _preId  = next.id;
   _preUrl = streamUrl(next.id);   // generate URL once — reused by main element
-  if(!_preAudio) { _preAudio = new Audio(); _preAudio.preload = 'auto'; }
-  _preAudio.volume = 0;
-  _preAudio.src = _preUrl;        // Chrome starts buffering immediately
+  if(_preLink) _preLink.remove();
+  _preLink = document.createElement('link');
+  _preLink.rel  = 'prefetch';
+  _preLink.as   = 'fetch';
+  _preLink.href = _preUrl;
+  document.head.appendChild(_preLink);
 }
 
 function _clearPreload() {
   _preId = ''; _preUrl = '';
-  if(_preAudio) { _preAudio.src = ''; }
+  if(_preLink) { _preLink.remove(); _preLink = null; }
 }
 
 // ─── PLAYBACK ───