Faster first paint: hero animation and minimap sizing

Three measured fixes from the browser performance data (LCP poor on 12% of visits, small layout shifts on the map grid):

• The animated Ambusher hero on the home page is a 625 KB GIF and was the slowest thing to paint. It now shows an 11 KB first-frame poster immediately and swaps to the full animation once that has loaded — no visual change, just a much faster first paint.
• All 4,145 map grid thumbnails and the minimap on every map page now carry explicit image dimensions and async decoding, so they reserve their space instead of shifting the page while loading.
• The animation swap is progressive enhancement: without JavaScript the poster stays visible, nothing breaks.