fix: update sound/notification globals in password branch + close AudioContext

Agent review findings:
- _soundEnabled/_notificationsEnabled not updated in the password-save
  early-return branch of saveSettings() — fixed
- AudioContext never closed after oscillator finishes — added osc.onended
  callback to ctx.close() preventing resource accumulation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/messages.js

@@ -379,6 +379,7 @@ function playNotificationSound(){
     gain.gain.setValueAtTime(0.3,ctx.currentTime);
     gain.gain.exponentialRampToValueAtTime(0.01,ctx.currentTime+0.3);
     osc.start(ctx.currentTime);osc.stop(ctx.currentTime+0.3);
+    osc.onended=()=>ctx.close();
   }catch(e){console.warn('Notification sound failed:',e);}
 }