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/tests/Syntax-TP/syntax03.md

@@ -0,0 +1,38 @@
+## SYNTAX03: There must be no double hyphen ('--') in position 3 and 4 of the domain name
+
+### Test case identifier
+**SYNTAX02** No double hyphen ('--') in position 3 and 4 of the domain name
+
+### Objective
+
+There must be no double hyphen ('--') in position 3 and 4 of the domain name,
+unless the domain name has the prefix 'xn--' which is used for
+internationalization.
+See section 5 of [RFC 3696](https://datatracker.ietf.org/doc/html/rfc3696#section-5),
+"Implications of internationalization".
+
+
+### Inputs
+
+The domain name to be tested.
+
+### Ordered description of steps to be taken to execute the test case
+
+1. Each label of the domain name of the test object is used as the input
+   for the validation.
+2. If any label in the domain name contains hyphens ('-') in position 3 and 4,
+   go to next step.
+3. Unless the prefix is 'xn', this test case fails.
+
+### Outcome(s)
+
+If any label in the domain name has a hyphen in position 3 and 4 of the label
+and the prefix is not 'xn', this test case fails.
+
+### Special procedural requirements
+
+None.
+
+### Intercase dependencies
+
+None.