feat: persist volume level in localStorage

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

src/main.js

@@ -162,7 +162,8 @@ window.addEventListener('load', () => {
     }, delay);
   });
 
-  S.audio.volume = 0.3;
+  const _savedVol = parseInt(localStorage.getItem('ns_volume') ?? '', 10);
+  setVolume(isNaN(_savedVol) ? 55 : _savedVol);
 
   let _wasPlayingBeforeHide = false;