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/Testing.md

@@ -0,0 +1,46 @@
+# Unit testing of Zonemaster-GUI
+
+## Setup
+
+This part assumed that you have a working NodeJS environment on Ubuntu. See
+the [build environment] document for further instructions.
+
+The tests for Zonemaster GUI are implemented using [playwright], to install the
+tests dependencies run the following.
+
+```sh
+npm run e2e:install-deps
+npm run e2e:install
+```
+
+This will install system dependencies as well as the browsers used to tests the
+GUI.
+
+## Running the tests
+
+To run the tests run the following command.
+
+```sh
+npm run e2e
+```
+
+If the appearance of the GUI changes some tests might fail as the baseline for
+visual comparison is no longer up to date. In this case it is required to update
+the snapshots. The following commands updates the snapshots while ensuring a
+similar environment than the one the CI runs in is used (Ubuntu 22.04 and UTC
+time).
+
+```sh
+docker run --rm --network host -v $(pwd):/work/ -w /work/ -it mcr.microsoft.com/playwright:v1.57.0-jammy /bin/bash
+npm run e2e:install
+npm run e2e:update
+exit
+```
+
+Note that this method may not always work: it could happen that the generated
+snapshots are incorrect and have slight differences (e.g. appears zoomed in/out,
+etc). In that case, repeat the above steps but outside of the Docker environment,
+and preferably on a machine that uses a similar OS/environment as the CI.
+
+[build environment]: https://github.com/zonemaster/zonemaster/blob/master/docs/internal/distrib-testing/Ubuntu-Node.js-build-environment.md
+[playwright]: https://playwright.dev/docs/intro