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/monitoredAreas.dart

@@ -80,7 +80,8 @@ class MonitoredAreasPage extends StatelessWidget {
                             children: [
                               new TileLayer(
                                   urlTemplate:
-                                      "https://tile.openstreetmap.org/{z}/{x}/{y}.png",
+                                      "https://cartodb-basemaps-{s}.global.ssl.fastly.net/light_all/{z}/{x}/{y}.png",
+                                  subdomains: ['a', 'b', 'c', 'd'],
                                   userAgentPackageName:
                                       'com.example.fires_flutter'),
                               new PolylineLayer(