feat: add full Zonemaster stack with Docker and Spanish UI

- Clone all 5 Zonemaster component repos (LDNS, Engine, CLI, Backend, GUI)
- Dockerfile.backend: 8-stage multi-stage build LDNS→Engine→CLI→Backend
- Dockerfile.gui: Astro static build served via nginx
- docker-compose.yml: backend (internal) + frontend (port 5353)
- nginx.conf: root redirects to /es/, /api/ proxied to backend
- zonemaster-gui/config.ts: defaultLanguage set to 'es' (Spanish)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

zonemaster/docs/internal/distrib-testing/Ubuntu-Node.js-build-environment.md

@@ -0,0 +1,42 @@
+# Ubuntu Node.js Build Environment
+
+The build process takes over 1GB of RAM. A machine with at least 2GB of RAM is recommended to build Zonemaster-GUI.
+
+The requirements to build the Zonemaster-GUI distribution zip file are Node.js
+and npm. Below are instructions to create such a build environment on Ubuntu.
+
+Node.js and npm are available from the [Node.js] official website. The required
+Node.js version is 18. The process has been tested on Ubuntu 22.04, which we use
+here.
+
+1. Make a clean installation of Ubuntu 22.04.
+
+2. Update the package database and install curl
+   ```sh
+   sudo apt-get update && sudo apt-get install curl
+   ```
+
+3. Install Node.js by using [NVM], a node version manager.
+   ```sh
+   curl -o- https://raw.githubusercontent.com/nvm-sh/nvm/v0.40.3/install.sh | bash
+   ```
+
+4. After installation, log out and log in again to handle [known issue], or just:
+
+   ```sh
+   source ~/.bashrc
+   ```
+
+5. Install the supported Node.js version
+   ```sh
+   nvm install 24
+   ```
+
+6. Switch to the previously installed version
+   ```sh
+   nvm use 24
+   ```
+
+[known issue]:                          https://github.com/nvm-sh/nvm#troubleshooting-on-linux
+[Node.js]:                              https://nodejs.org/en
+[NVM]:                                  https://github.com/nvm-sh/nvm