Feature Request: Add environment variable for Base Path / Subpath support (e.g., BASE_PATH)

Is your feature request related to a problem? Please describe.

Currently, Nginx Proxy Manager (NPM) assumes it is always running on the root directory (/) of a domain or subdomain. When trying to host NPM behind another reverse proxy (like YARP, Traefik, or an upstream Nginx) under a specific subpath (e.g., https://mydomain.com/npm), the Admin UI completely breaks.

This happens because all asset paths (JS, CSS) and API calls are hardcoded to the root (e.g., /assets/main.js or /api/). The browser tries to fetch them from the root of the main proxy instead of the subpath, resulting in 404 Not Found errors.

Describe the solution you'd like

It would be amazing to have an environment variable—similar to Kibana's SERVER_BASEPATH or Grafana's root_url—such as BASE_PATH or APP_BASE_URL.

When this variable is set (e.g., BASE_PATH=/npm):

1. The frontend should prepend this path to all static asset requests.
2. The UI should send all API requests to /<BASE_PATH>/api/... instead of /api/....
3. The backend routing should respect this prefix.

Describe alternatives you've considered

• Using subdomains: While using npm.mydomain.com works, it is not always feasible due to corporate network constraints, SSL certificate limitations, or strict domain policies where only a single domain with subpaths is allowed.
• URL Rewriting / Response Transformation: Trying to rewrite the response body dynamically on the upstream proxy to inject the subpath is incredibly brittle and breaks easily with new updates.

Additional context

Many popular DevOps and containerized tools (like Kibana, Grafana, Portainer, and Gitea) support base path configuration natively out of the box via environment variables. Adding this feature would greatly improve NPM's flexibility in complex network architectures and enterprise environments.

The issue names no files, tests, or entry points. Start by locating where the frontend builds asset and API URLs, then find the backend routing configuration; verify how both layers can consume BASE_PATH. Done means the Admin UI assets, API requests, and backend routes work correctly under a configured subpath without breaking root deployment.