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/internal/requirements/ExistingZCFeatures.md

@@ -0,0 +1,18 @@
+# ZoneCheck Features
+
+ * Powerful XML based configuration file (allowing changes in test severity,
+order, zone of application, ...)
+ * Does not depend on policies
+ * Fine grained test selection (by test, by categories, by zones)
+ * Full IPv6 support (connectivity and AAAA records)
+ * Supports several input/output interfaces such as CLI, CGI, GUI, inetd
+ * Dedicated mode for use inside shell scripts
+ * Batch mode available (ideal when dealing with several domains)
+ * Use of stylesheets for easy integration and javascript for enhancement only
+ * Generates reports either by severity or by hosts
+ * I18N and L10N support (available: French, English)
+ * Multi-threaded application in order to cut down checking time
+ * Extensible: new tests, new interfaces, new reports, ...
+ * Exception and cache mechanisms to simplify test writing
+ * Open source
+