Add an Admin link to the web app sidebar

Problem

Self-hosted deployments expose the Django admin at /server/admin, but the web application's sidebar does not provide a discoverable entry point. Users must know or manually type the URL.

Proposed change

Add an Admin item to the sidebar menu in src/interface/web/app/components/appSidebar/appSidebar.tsx, alongside Settings, linking to /server/admin.

The link should remain relative so it works for localhost, reverse-proxy deployments, and custom domains.

Validation

I tested this in a local Docker self-hosted deployment. The added sidebar item navigates successfully to the existing Django admin endpoint. This is a small UI-only change and does not require API or backend changes.

Thanks for considering it!