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/syntax08.md

@@ -0,0 +1,49 @@
+## SYNTAX08: MX name must have a valid hostname
+
+### Test case identifier
+**SYNTAX08** The MX record name must be a valid hostname
+
+### Objective
+
+The MX record names used for delivering mail for a domain name address
+must be valid hostnames according to the rules defined in
+[RFC 952](https://datatracker.ietf.org/doc/html/rfc952),
+in section 2.1 in [RFC 1123](https://datatracker.ietf.org/doc/html/rfc1123#section-2.1),
+section 11 in [RFC 2182](https://datatracker.ietf.org/doc/html/rfc2181#section-11) and
+section 2 and 5 in [RFC 3696](https://datatracker.ietf.org/doc/html/rfc3696#section-2).
+Newer RFCs may override some rules defined in earlier documents. The MX
+records use of "Domain Names" is described in section 2.3.5  of
+[RFC 5321](https://datatracker.ietf.org/doc/html/rfc5321#section-2.3.5).
+
+### Inputs
+
+The hostnames to be tested. The hostnames comes from looking up the MX record
+for the domain being tested.
+
+### Ordered description of steps to be taken to execute the test case
+
+1. Query for the MX record of the domain name.
+2. For each hostname of the MX records found:
+3. If any label in the hostname does not contain a-z or 0-9 this test case
+   fails.
+4. If any label of the hostname is longer than 63 characters, this test case
+   fails.
+5. If the hostname is longer than 255 characters including separators, this
+   test case fails.
+6. If the rightmost label (the TLD) contains only digits, this test case
+   fails.
+7. If there is a hyphen ('-') in position 3 and 4 of the label, and the prefix
+   is not xn (used for internationalization), this test case fails.
+
+### Outcome(s)
+
+If any of the steps 3 to 7 in the ordered description of this test case fails,
+the whole test case fails.
+
+### Special procedural requirements
+
+None.
+
+### Intercase dependencies
+
+None.