Add leaflet-google-mutant@0.16.0 (exact/locked version) as a proper bridge
between the Google Maps JavaScript API and Leaflet. The old mt{s}.google.com
tile URL approach was unofficial and produced empty tiles.
Traffic layer now renders via createLayerComponent + googleMutant, loaded only
after the Maps JS API script is injected and ready (keyed off NEXT_PUBLIC_GOOGLE_MAPS_API_KEY).
Added leaflet-google-mutant to transpilePackages in next.config.mjs.