Fail the build on missing network config outside local #418

Expected outcome

A build for testnet or mainnet with missing or invalid PUBLIC_STELLAR_* variables fails with a message naming them, instead of pointing the app at localhost.

Acceptance criteria

• When PUBLIC_STELLAR_NETWORK is set to anything other than local and validation fails, the build throws an error that names the invalid variables
• A local build without the variables still falls back to localhost, so CI keeps passing
• One test for each case