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>

frontend/.env.example

@@ -14,3 +14,8 @@
 # Only set these if you need to connect to backend services directly
 # NEXT_PUBLIC_BACKEND_BASE_URL="http://localhost:8001"
 # NEXT_PUBLIC_LANGGRAPH_BASE_URL="http://localhost:2024"
+
+# Server-only Gateway wiring used by SSR (auth checks, /api/* rewrites).
+# Defaults to localhost — only override for non-local deployments.
+# DEER_FLOW_INTERNAL_GATEWAY_BASE_URL="http://localhost:8001"
+# DEER_FLOW_TRUSTED_ORIGINS="http://localhost:3000,http://localhost:2026"