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/syntax05.md

@@ -0,0 +1,41 @@
+## SYNTAX05: Misuse of '@' character in the SOA RNAME field
+
+### Test case identifier
+**SYNTAX05** There must be no misused '@' character in the SOA RNAME field
+
+### Objective
+
+The SOA RNAME field does not allow the '@' characters to be used for
+describing a mailbox. The first dot ('.') is thus translated into the
+'@' character. This is a common mistake. The rules are defined in
+[RFC 1035](https://datatracker.ietf.org/doc/html/rfc1035).
+
+### Inputs
+
+The domain name to be tested.
+
+### Ordered description of steps to be taken to execute the test case
+
+1. Obtain a set of name server IP addresses using [Method4] and [Method5].
+2. Create a SOA query for the zone.
+3. Send the SOA query over UDP to each name server IP address until a
+   response is received or until the set is exhausted.
+4. Check if the RNAME field contains a '@' character.
+
+
+### Outcome(s)
+
+If there is any '@' character in any SOA/RNAME field, this test case fails.
+
+### Special procedural requirements
+
+None.
+
+### Intercase dependencies
+
+The de-escaped output from this test is used by [SYNTAX08](syntax08.md).
+
+-------
+
+[Method4]: ../Methods.md#method-4-obtain-glue-address-records-from-parent
+[Method5]: ../Methods.md#method-5-obtain-the-name-server-address-records-from-child