revert: remove leaflet.gridlayer.googlemutant — incompatible with Next.js 15 bundler

The package consistently throws 'L.GridLayer.GoogleMutant is not a constructor'
due to L-instance conflicts in the webpack bundle, despite multiple workaround
attempts. Removed package, transpilePackages entry, type stub, env var, and all
related component code. Traffic overlay dropped; geocoding (backend) unaffected.

drb-frontend/next.config.mjs

@@ -1,7 +1,7 @@
 /** @type {import('next').NextConfig} */
 const nextConfig = {
   output: "standalone",
-  transpilePackages: ["leaflet", "react-leaflet", "leaflet.gridlayer.googlemutant"],
+  transpilePackages: ["leaflet", "react-leaflet"],
 };
 
 export default nextConfig;