Move relayer gateway defaults into network configuration

[collinsadi]

frontend/src/lib/relayerMarket.ts hardcodes DEFAULT_GATEWAYS = ["http://127.0.0.1:8787"] and a 720-ledger job deadline inline, separate from where the app's other network endpoints are configured.

Move gateway endpoints and deadlines into the per-network configuration with env overrides.

Acceptance criteria:

• Gateway defaults come from the same config source as RPC endpoints, per network.
• A localhost default never ships in a production build.
• The env vars are documented in frontend/.env.example.