Geocodes an address string via Google Maps API.
Returns {lat, lng, source:'google'} or null on failure / missing key.
Aborts after 800 ms to avoid blocking address submission.
API key is read from CDN remoteKeys (RemoteKeyKeys.googleMapsKey).
Saves a new address for a phone number.
Sets muToken + x-idempotency-key on global client headers (mirrors RN),
then removes x-idempotency-key after the call to avoid polluting other requests.