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/specifications/README.md

@@ -0,0 +1,9 @@
+# Specifications
+
+* [Test Cases](tests/README.md): contains the specifications of the *Test Cases*
+  that the Zonemaster implementation is based on.
+* [Test types](test-types/README.md): contains the specification of *Test Types*,
+  currently only for undelegated tests.
+* [Test zones](test-zones/README.md): contains the specifications of *Test Zones*
+  for the verification of Test Case implementation.
+