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/package.json

@@ -14,8 +14,7 @@
     "react-dom": "^18.3.0",
     "firebase": "^10.12.0",
     "leaflet": "^1.9.4",
-    "leaflet.gridlayer.googlemutant": "0.16.0",
-    "react-leaflet": "^4.2.1"
+"react-leaflet": "^4.2.1"
   },
   "devDependencies": {
     "typescript": "^5.4.0",