World map: switch to CARTO Voyager basemap, add carto_key variable

The default Geomap basemap already works without a key, but the operator wants to use CARTO's voyager tileset explicitly, which requires an API key. Added a hidden constant variable "carto_key" (empty placeholder) referenced in the basemap URL - fill in its "current.value" in this file once the key is available.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01EV6SQsz6pc6saCHMjNtFN9