Switch map tiles from CARTO to Stadia Maps dark

CARTO now requires an API key; Stadia Maps alidade_smooth_dark is free without one.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

static/index.html

@@ -253,8 +253,8 @@
 
 <script>
 const map = L.map('map', { zoomControl: true }).setView([37.7749, -122.4194], 11);
-L.tileLayer('https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png', {
-  attribution: '© OpenStreetMap contributors © CARTO', maxZoom: 19, subdomains: 'abcd'
+L.tileLayer('https://tiles.stadiamaps.com/tiles/alidade_smooth_dark/{z}/{x}/{y}{r}.png', {
+  attribution: '© <a href="https://stadiamaps.com/">Stadia Maps</a> © <a href="https://openmaptiles.org/">OpenMapTiles</a> © <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>', maxZoom: 20
 }).addTo(map);
 
 let allRides = [];