fix(deploy): unify backend URL policy

docker/frontend-entrypoint.sh

@@ -2,19 +2,10 @@
 set -eu
 
 backend_base_url=${BACKEND_BASE_URL-/api}
-case "$backend_base_url" in
-  ""|/|/api|/api/) ;;
-  http://*|https://*)
-    if printf '%s' "$backend_base_url" | grep -Eq '[[:space:]]|^https?://[^/]*@'; then
-      echo "Invalid BACKEND_BASE_URL: credentials and whitespace are not allowed" >&2
-      exit 2
-    fi
-    ;;
-  *)
-    echo "Invalid BACKEND_BASE_URL: use empty/root, /api, or an absolute http(s) URL" >&2
-    exit 2
-    ;;
-esac
+if ! /usr/local/bin/validate-backend-url "$backend_base_url"; then
+  echo "Invalid BACKEND_BASE_URL: use empty/root, /api, or a valid http(s) base without credentials, query, or fragment" >&2
+  exit 2
+fi
 runtime_config=$(jq -cn --arg backend_base_url "$backend_base_url" \
   '{backendBaseUrl: $backend_base_url}')
 printf 'window.__THOTHII_CONFIG__ = %s;\n' "$runtime_config" \