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/using/gui/README.md

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+# Using the GUI
+
+* The [GUI API][API]
+
+[API]: api.md

zonemaster/docs/public/using/gui/api.md

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+# API
+
+
+The Zonemaster GUI provides an API to access specific resource and perform
+specific tasks by adding a path to the Zonemaster GUI base URL, e.g.
+`https://zonemaster.net/`.
+
+For the paths the following keys are used:
+
+* `<lang>`: A [Language Code][Language Codes]
+* `<test-id>`: A string of 16 characters in `0-9a-f` (hexadecimal)
+* `<domain>`: A domain name to be tested (temporarily not supported)
+
+
+The supported paths are the following:
+
+* `/<lang>/result/<test-id>`: access the result page with results for the test
+  with `<test-id>` in the selected language, e.g.
+  `https://zonemaster.net/en/result/7e363fb606343397`.
+* `/<lang>/faq`: access the FAQ in the selected language, e.g.
+  `https://zonemaster.net/en/faq`.
+* `/api`: With JSON RPC call access the Zonemaster [Backend RPC-API], e.g.
+  `https://zonemaster.net/api`.
+
+The GUI will redirect from `<path>` (e.g. `https://zonemaster.net/`) to
+`<lang>/<path>` (`https://zonemaster.net/en/`) automatically using the
+user browser language, if supported and configured by the installation.
+Else it uses the default language defined in Zonemaster-GUI.
+
+The following paths are temporarily not supported:
+
+* `<lang>/run-test`: access the input form.
+* `<lang>/run-test/<domain>`: populate the input form with `<domain>`.
+
+## Language Codes
+
+Zonemaster uses [ISO 639-1] two-letter language codes, in lower case. A default
+installation of Zonemaster-GUI currently has support for the following language
+codes and languages:
+
+* `da` for Danish language
+* `en` for English language, the default language in a default installation
+* `es` for Spanish language
+* `fi` for Finnish language
+* `fr` for French language
+* `nb` for Norwegian language
+* `sl` for Slovenian language
+* `sv` for Swedish language
+
+A non-default installation may have support for only some of the languages or
+maybe other languages.
+
+
+[ISO 639-1]:                                   https://en.wikipedia.org/wiki/List_of_ISO_639-1_codes
+[Language Codes]:                              #language-codes
+[Backend RPC-API]:                             ../backend/rpcapi-reference.md