Add support for custom Nginx maintenance templates (NGINX_UI_NGINX_MAINTENANCE_TEMPLATE / Settings > Nginx)

[enwikuna]

• Add/use environment variable NGINX_UI_NGINX_MAINTENANCE_TEMPLATE to point the app to a custom maintenance HTML template used when maintenance mode is enabled.
• Allow the maintenance template path to be configured from the web UI as well: Settings > Nginx.
• If a custom path is provided (env var or UI), the app will load that file from inside the container. If not provided, the built-in default template is used.
• Document how to provide custom templates by mounting a host directory into the container:
  • Example mount: ./nginx:/etc/nginx
  • Create a maintenance folder inside that mount: ./nginx/maintenance
  • Put your template file there, e.g. ./nginx/maintenance/maintenance.html
  • Set the template path to the absolute path inside the container:
    NGINX_UI_NGINX_MAINTENANCE_TEMPLATE=/etc/nginx/maintenance/maintenance.html
• Notes:
  • The path must be the absolute path inside the container and the file must be readable by the nginx process (e.g., mode 644).
  • After changing the env var or updating templates (or changing the setting in Settings > Nginx), restart the container or reload nginx for changes to take effect.
  • You can store multiple templates under ./nginx/maintenance and switch between them by updating the env var or the UI setting.
• Include example docker-compose snippet in docs:
  volumes:
  • ./nginx:/etc/nginx
    environment:
  • NGINX_UI_NGINX_MAINTENANCE_TEMPLATE=/etc/nginx/maintenance/maintenance.html

This commit adds configuration surface and documentation for using custom maintenance templates via environment variable or the UI.

services:
  nginx-ui:
    image: nginx-ui-fixed:local
    container_name: nginx-ui
    restart: always
    stdin_open: true
    tty: true
    environment:
      - TZ=Europe/Berlin
      - GIN_MODE=release
      - NGINX_UI_NGINX_MAINTENANCE_TEMPLATE=enwikuna-maintenance.html

Kindly ask you to test and merge our changes into production line.