Visitor geolocation already flows via the Cloudflare CF-IPLatitude /
CF-IPLongitude headers into the session; MapLive centers the map on
the visitor's location (or DFW as fallback). Add a banner that appears
above the map when those coords are outside the CONUS bbox
(lat 24.5-49.5, lon -125 to -66.5), letting overseas visitors know
propagation data won't be available at their location yet.
Map still centers on the visitor's actual location — this is a note,
not a re-center. Banner is fully absent from the DOM when not needed
(:if conditional).