Installer: create gateway.env pre-populated with empty provider key variables

[gregjkal]

Summary

The installer (or first boot) should create gateway.env with every supported provider's API key variable already present and empty, so the only thing a user has to do is open the file and paste their keys.

Details

From the #cppa-wg21-org design discussion on 2026-09-09:

• Today the gateway loads a single config-sibling gateway.env at boot (crates/gateway/src/env_file.rs) and the Secrets view edits it.
• The variable list should stay in sync with the provider catalog (Gateway: updatable provider/model catalog file #25), so adding a provider adds its key slot.
• The Secrets view should show these empty slots so keys can also be pasted from the UI.

Related

• Provider catalog UI: Gateway: closed-weight frontier provider catalog with per-provider API key UI #24
• Provider catalog file: Gateway: updatable provider/model catalog file #25