Provide env variable alternative to `testMode`

`testMode` defaulting to true is good day 1, but if you're already set up with Resend it's a bit awkward to keep `true` forever, and likely wants to have different behavior in dev/staging/CI than prod

Start by locating the TypeScript configuration and environment-variable handling for `testMode`. Define how the environment alternative should control behavior in development, staging, CI, and production, then verify that the existing default remains unchanged when the variable is absent.

Written by the indexing model from the issue text.