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/configuration/gui/configuring-using-config-json.md

@@ -0,0 +1,66 @@
+# Run-time configuring Zonemaster-GUI using "config.json"
+
+The `config.json` configuration file can be used for *run-time* settings of the
+Zonemaster-GUI, i.e. no rebuild of the installation package is needed.
+
+Currently, the only supported setting is creating, updating or removing a message
+banner. Other types of configuration require a rebuild of the GUI, and are covered
+in sibling documents of this document.
+
+## Finding "config.json"
+
+In the source tree the file is found as `static/config.json` relative to the
+root of the Git tree.
+
+In a default installation, the file is found as
+`/var/www/html/zonemaster-web-gui/dist/config.json`.
+
+## Reloading GUI after update
+
+When the Web browser reloads the GUI, e.g. after running a new test, the
+changes are displayed by the GUI.
+
+## Default "config.json" file
+
+The `config.json` file contains configuration that is loaded at runtime.
+Currently, it includes settings for displaying banner messages in different
+languages:
+
+```json
+{
+    "msgBanner": {
+        "en": "",
+        "sv": "",
+        "da": "",
+        "es": "",
+        "fi": "",
+        "fr": "",
+        "nb": ""
+    }
+}
+```
+
+### Configuration Options
+
+* **msgBanner**: An object containing message banner text for each supported
+  language. These messages can be used to display important announcements or
+  notifications to users in their preferred language.
+
+The message banner is shown just below the top menu on all pages.
+  
+The string is set for each language, and may contain HTML code such as `<br>` or
+even `<a href>` if necessary. Take note not to break quoting as JSON requires
+quoting with `""` around the string.
+
+To display a message banner, simply add the message text to the corresponding
+language key. E.g.:
+
+```json
+{
+    "msgBanner": {
+        "en": "Scheduled system maintenance on 2025-12-09 from 10:00 to 12:00 UTC.",
+        "sv": "Planerat systemunderhåll den 9 december 2025 kl. 10.00–12.00 UTC."
+    }
+}
+```
+