Add a server instance selector and drop env vars

src/env.ts

@@ -1,10 +0,0 @@
-if (typeof process.env.REACT_APP_BUILD !== 'string') {
-  throw new Error('ENV: No env provided')
-}
-if (!['dev', 'staging', 'prod'].includes(process.env.REACT_APP_BUILD)) {
-  throw new Error(
-    `ENV: Env must be "dev", "staging", or "prod," got "${process.env.REACT_APP_BUILD}"`,
-  )
-}
-
-export const BUILD = process.env.REACT_APP_BUILD