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/public/upgrading/gui.md

@@ -0,0 +1,26 @@
+# Upgrade GUI
+
+## Overview
+
+This document documents manual steps needed to keep customizations, if any, from
+one version of Zonemaster-GUI to a later version. If no such customizations exist
+nothing special has to be done.
+
+Migrations:
+ * [pre-v5.0.0 to v5.0.0](#migrating-pre-v5-to-v5)
+
+## Find version of GUI
+
+Go to the start web page of Zonemaster. In the lower left corner you can find the
+the Zonemaster-GUI version.
+
+## Migrating pre-v5 to v5
+
+If `assets/app.config.json` is found in installed Zonemaster-GUI go to the
+following documents to migrates its content:
+* [Configuring Zonemaster GUI using "config.json"]
+* [Configuring Zonemaster GUI using "config.ts"]
+
+
+[Configuring Zonemaster GUI using "config.json"]:                    ../configuration/gui/configuring-using-config-json.md
+[Configuring Zonemaster GUI using "config.ts"]:                      ../configuration/gui/configuring-using-config-ts.md