fix: switch basemap from demo tiles to CARTO Positron

Replace MapLibre demo tiles (country outlines only) with CARTO Positron
basemap showing roads, cities, and place labels so users can locate
their programme areas.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

frontend/js/map.js

@@ -3,6 +3,8 @@
  * Manages the AOI definition map (Define Area page) and the results map.
  */
 
+const MAP_STYLE = 'https://basemaps.cartocdn.com/gl/positron-gl-style/style.json';
+
 /* ── AOI Map ─────────────────────────────────────────────── */
 
 let _aoiMap = null;
@@ -19,7 +21,7 @@ export function initAoiMap(containerId, onAoiChange) {
 
   _aoiMap = new maplibregl.Map({
     container: containerId,
-    style: 'https://demotiles.maplibre.org/style.json',
+    style: MAP_STYLE,
     center: [37.0, 3.0], // East Africa default
     zoom: 4,
   });
@@ -145,7 +147,7 @@ let _resultsMap = null;
 export function initResultsMap(containerId, bbox) {
   _resultsMap = new maplibregl.Map({
     container: containerId,
-    style: 'https://demotiles.maplibre.org/style.json',
+    style: MAP_STYLE,
     bounds: [[bbox[0], bbox[1]], [bbox[2], bbox[3]]],
     fitBoundsOptions: { padding: 60 },
   });