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-gui/docs/UI.md

@@ -0,0 +1,54 @@
+# Zonemaster UI
+
+Zonemaster UI (Zonemaster-GUI) is a static website built with [Astro], using
+[Svelte] for interactive components and [Vite] for internationalization/translation
+(i18n).
+
+## Standalone Components
+
+Zonemaster UI also provides a set of standalone web components that can be
+reused in other projects. Currently, there are two available:
+
+- `<zm-domain-test>` – for initiating domain tests
+- `<zm-program-versions>` – displays the current versions of Zonemaster
+  components
+
+These components are framework-agnostic and can be embedded in other websites or
+apps as needed.
+
+```html
+<zm-domain-test></zm-domain-test>
+<zm-program-versions></zm-program-versions>
+
+<script type="module">
+    import '@zonemaster/ui';
+</script>
+```
+
+## Headless Mode
+
+Zonemaster UI can also be used in **headless mode**, where it acts as a
+JavaScript API layer between the Zonemaster backend and your own frontend.
+
+This is useful if you want to:
+
+- Build your own custom UI
+- Integrate Zonemaster functionality into another application or CLI tool
+
+In headless mode, the UI logic is decoupled from the visual components, allowing
+you to access core functionality programmatically through JavaScript.
+
+```ts
+import { TestAgent } from '@zonemaster/ui';
+
+TestAgent.subscribe((state, context) => {
+    console.log(state, context);
+});
+
+TestAgent.transition('START', { domain: 'example.com' });
+```
+
+
+[Astro]:      https://astro.build/
+[Svelte]:     https://svelte.dev/
+[Vite]:       https://vite.dev/