Fix: Use CartoDB Positron for osmcGrey layer (reliable b/n tiles)

CartoDB Positron (light_all) is a modern, reliable replacement for the
deprecated Stamen Toner and tiles.wmflabs.org. It provides a clean
grayscale/minimalist map style with excellent availability.

- Updated FireMapLayer.osmcGrey to use CartoDB Positron
- Updated monitoredAreas TileLayer to use CartoDB Positron
- Keeps subdomains configuration for load distribution
- Fixes: Unreliable tile server DNS issues

lib/genericMap.dart

@@ -192,7 +192,9 @@ class _genericMapState extends State<genericMap> {
                   'https://{s}.tile.openstreetmap.fr/hot/{z}/{x}/{y}.png';
               break;
             case FireMapLayer.osmcGrey:
-              baseLayer = 'https://tile.openstreetmap.org/{z}/{x}/{y}.png';
+              baseLayer =
+                  'https://cartodb-basemaps-{s}.global.ssl.fastly.net/light_all/{z}/{x}/{y}.png';
+              subdomains = ['a', 'b', 'c', 'd'];
               break;
             case FireMapLayer.esri:
               baseLayer =