Browser-safe Stellar values use the PUBLIC_ prefix and are loaded by
src/lib/env.ts. The Node indexer validates the same deployment values at
startup through server/src/config/stellar.ts. Server secrets live in the
separate server/src/config/secrets.ts module; browser code must never import
that module or expose MONGODB_URI, challenge secrets, or private keys.
Use .env.example for development/testnet and env.mainnet.example for
mainnet. A deployment must set network, passphrase, RPC, Horizon, PromptHash
contract, native SAC, and (when simulation is used) the simulation account as
one consistent set. Invalid URLs and StrKeys fail startup instead of silently
falling back to a different network.