Add missing env

Author: chrissearle

frontend/README.md

@@ -31,6 +31,7 @@ Assuming we will build a docker container - add to [frontend action](../.github/
 
 | Environment Variable       | Description                                              | Default                                        |
 |----------------------------|----------------------------------------------------------|------------------------------------------------|
+| `CUPCAKE_BACKEND`          | Backend base URL for the server-side proxy               | `http://127.0.0.1:8080` (dev) / `https://cupcake-backend.java.no` (prod) |
 | `CUPCAKE_FRONTEND`         | Hostname the dev server allows (Vite `allowedHosts`)     | `localhost`                                    |
 | `NUXT_PUBLIC_OIDC_AUTHORITY` | OIDC authority URL (e.g. Keycloak realm URL)           | `https://auth.home.chrissearle.org/realms/HA12` |
 | `NUXT_PUBLIC_OIDC_CLIENT_ID` | OIDC client ID                                         | `cupcake-client`                               |