feat: persist volume level in localStorage

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

src/player.js

@@ -765,6 +765,7 @@ export function setVolume(v) {
   if (absAudio) absAudio.volume = vol;
   document.querySelectorAll('.volume-slider-sync').forEach(el => { el.value = v; });
   if (HAS_BRIDGE) _nsBridge('setVolume', vol);
+  try { localStorage.setItem('ns_volume', v); } catch (_) {}
 }
 
 // ─── MEDIA SESSION ────────────────────────────────────────────────────────