Fix missing .png extension in tile URL

vendor/clouds3d.js

@@ -319,7 +319,7 @@ function fetchTileImage(tileX, tileY) {
         img.onload = function() { resolve(img); };
         img.onerror = function() { reject(new Error('tile fetch failed')); };
         // Carto/OSM-style format: {z}/{x}/{y}.png via local proxy
-        img.src = GROUND_TILE_URL + GROUND_TILE_Z + '/' + tileX + '/' + tileY;
+        img.src = GROUND_TILE_URL + GROUND_TILE_Z + '/' + tileX + '/' + tileY + '.png';
     });
 }