fix(frontend): restore localhost fallback for getGatewayConfig in prod mode (#2705) (#2718)

* fix(frontend): unify gateway-config localhost fallback for prod (#2705)

`getGatewayConfig()` only fell back to localhost defaults when
`NODE_ENV === "development"`, while `next.config.js` always falls back
to `127.0.0.1:8001`. Running `make start` (which sets NODE_ENV=production
via `next start`) without `DEER_FLOW_INTERNAL_GATEWAY_BASE_URL` /
`DEER_FLOW_TRUSTED_ORIGINS` therefore caused zod to throw inside SSR
layouts and surfaced as a 500.

Drop the NODE_ENV gating and use localhost defaults everywhere — the
"force explicit config in prod" intent should be enforced by deployment
templates (docker-compose already sets both vars), not by request-time
crashes. Document the two vars in both .env.example files and add unit
coverage for the dev/prod env-unset paths.

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

* Update internalGatewayUrl in gateway config tests

---------

Co-authored-by: Willem Jiang <willem.jiang@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.env.example

@@ -48,3 +48,14 @@ INFOQUEST_API_KEY=your-infoquest-api-key
 
 # Set to "false" to disable Swagger UI, ReDoc, and OpenAPI schema in production
 # GATEWAY_ENABLE_DOCS=false
+
+# ── Frontend SSR → Gateway wiring ─────────────────────────────────────────────
+# The Next.js server uses these to reach the Gateway during SSR (auth checks,
+# /api/* rewrites). They default to localhost values that match `make dev` and
+# `make start`, so most local users do not need to set them.
+#
+# Override only when the Gateway is not on localhost:8001 (e.g. when the
+# frontend and gateway run on different hosts, in containers with a service
+# alias, or behind a different port). docker-compose already sets these.
+# DEER_FLOW_INTERNAL_GATEWAY_BASE_URL=http://localhost:8001
+# DEER_FLOW_TRUSTED_ORIGINS=http://localhost:3000,http://localhost:2026