Switch to ESRI World_Street_Map via local CORS proxy

Carto Voyager was returning 'API Key required' content for the
user (likely rate-limited or IP-blocked by Carto). Switched to
ESRI World_Street_Map which has CORS-friendly tiles with plenty
of place names and street labels.

URL format is different from Carto: {z}/{y}/{x}.jpg (y before x,
jpg not png). Updated the build to match. The local /esri/
proxy in default.conf handles the upstream CORS-incompatibility
by adding permissive CORS headers and a proper User-Agent.

vendor/clouds3d.js

@@ -284,7 +284,7 @@ function init(canvas) {
 // mosaic centre always corresponds to UV (0.5, 0.5).
 // Tile source: Carto basemaps Voyager style (with place names).
 // Free, CORS-enabled, OSM-derived. URL format {z}/{x}/{y}.png.
-var GROUND_TILE_URL = '/carto/voyager/';
+var GROUND_TILE_URL = '/esri/streets/';
 var GROUND_TILE_Z = 11;
 var GROUND_TILE_SIZE = 256;
 // Fetch direction: which way the tile corridor is oriented.
@@ -318,8 +318,9 @@ function fetchTileImage(tileX, tileY) {
         img.crossOrigin = 'anonymous';
         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 + '.png';
+        // ESRI World_Street_Map format: {z}/{y}/{x}.jpg via local
+        // proxy. (y before x, jpg not png)
+        img.src = GROUND_TILE_URL + GROUND_TILE_Z + '/' + tileY + '/' + tileX + '.jpg';
     });
 }