Add screenshot to README

README.md

@@ -4,6 +4,8 @@ A self-contained Go server that receives GPS location data from the
 [Colota Android app](https://colota.app), stores it in an SQLite3 database,
 and serves a web frontend for browsing trips on a map.
 
+![Breadcrumb screenshot](docs/screenshot.png)
+
 ## Quick Start
 
 ```bash

static/index.html

@@ -511,9 +511,9 @@
   // ── Map ───────────────────────────────────────────────────────────────────────
   const map = L.map('map', { zoomControl: true }).setView([20, 0], 2);
 
-  L.tileLayer('https://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}{r}.png', {
+  L.tileLayer('https://tiles.stadiamaps.com/tiles/alidade_smooth_dark/{z}/{x}/{y}{r}.png', {
-    attribution: '&copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a> &copy; <a href="https://carto.com/">CARTO</a>',
+    attribution: '&copy; <a href="https://stadiamaps.com/">Stadia Maps</a> &copy; <a href="https://openmaptiles.org/">OpenMapTiles</a> &copy; <a href="https://www.openstreetmap.org/copyright">OpenStreetMap</a>',
-    maxZoom: 19
+    maxZoom: 20
   }).addTo(map);
 
   let trackLayer = null, markerStart = null, markerEnd = null, pointsLayer = null;