Build a Sokoban puzzle game as a static web page in this directory: index.html, style.css, game.js. Requirements: 8 hand-crafted levels of increasing difficulty; arrow-key movement with box pushing; U undoes one move, R restarts the level; move and push counters; win detection with a level-complete overlay and auto-advance; a level-select strip showing completed levels; retro tile look drawn in pure CSS (walls, floor, goals, boxes, player) with no external assets or build step; everything must work by opening index.html.