Extract YAML validation to separate module

- Created yaml-validator.js with pure validation functions
- Removed redundant checkYAMLFormatting function (rely on js-yaml)
- Fixed function shadowing issues by using window._validateYAMLPure
- Updated nginx config to serve JS files with correct MIME type
- Improved error reporting with line/column numbers from js-yaml

docker-compose.yml

@@ -15,6 +15,7 @@ services:
       # Hot reload frontend (optional, for development)
       - ./index.html:/usr/share/nginx/html/index.html:ro,z
       - ./manage.html:/usr/share/nginx/html/manage.html:ro,z
+      - ./yaml-validator.js:/usr/share/nginx/html/yaml-validator.js:ro,z
     environment:
       # Set working directory for database
       - PWD=/app/data