fix: make ZERO_QUERY_URL portable across all environments

- Default to Docker service networking (http://frontend:3000) in both
  compose files — works when all services run in Docker
- Add extra_hosts (host.docker.internal:host-gateway) for cross-platform
  host access when running frontend outside Docker
- Document override examples in .env.example for local dev and
  custom domain setups

docker/docker-compose.dev.yml

@@ -173,6 +173,8 @@ services:
     image: rocicorp/zero:0.26.2
     ports:
       - "${ZERO_CACHE_PORT:-4848}:4848"
+    extra_hosts:
+      - "host.docker.internal:host-gateway"
     depends_on:
       db:
         condition: service_healthy
@@ -182,7 +184,7 @@ services:
       - ZERO_CHANGE_DB=${ZERO_CHANGE_DB:-postgresql://${DB_USER:-postgres}:${DB_PASSWORD:-postgres}@${DB_HOST:-db}:${DB_PORT:-5432}/${DB_NAME:-surfsense}?sslmode=${DB_SSLMODE:-disable}}
       - ZERO_REPLICA_FILE=/data/zero.db
       - ZERO_ADMIN_PASSWORD=${ZERO_ADMIN_PASSWORD:-surfsense-zero-admin}
-      - ZERO_QUERY_URL=${ZERO_QUERY_URL:-http://host.docker.internal:3000/api/zero/query}
+      - ZERO_QUERY_URL=${ZERO_QUERY_URL:-http://frontend:3000/api/zero/query}
     volumes:
       - zero_cache_data:/data
     restart: unless-stopped