Lower the lobby music to 70% (0.4 -> 0.28)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/ui.js

@@ -105,7 +105,7 @@ export function initUI({
   // audio autoplay until the user interacts, so it's primed on the first gesture;
   // after that it just follows the player state via syncPlayState() -> updateLobby().
   // Track: Alex Morgan — "Peaceful Cafe Jazz" (Pixabay, royalty-free).
-  const LOBBY_VOL = 0.4;
+  const LOBBY_VOL = 0.28; // 70% of the original 0.4
   const lobby = new Audio("/static/lobby.mp3");
   lobby.loop = true;
   lobby.preload = "auto";