feat: configurable UI base path via BASE_PATH build arg

Allows serving the UI under a subpath (e.g. /ui) instead of root.

- Vite config reads BASE_PATH env var, sets it as the asset base
  and injects __BASE_PATH__ for runtime use
- React Router uses basename from __BASE_PATH__ for client-side routing
- Nginx config uses ${BASE_PATH} placeholder, replaced at build time
- Dockerfile accepts BASE_PATH build arg (default: /)
- Woodpecker docker pipeline passes BASE_PATH=/ui

.woodpecker/docker.yaml

@@ -22,6 +22,8 @@ steps:
       repo: git.unkin.net/unkin/artifactapi-ui
       dockerfile: ui/Dockerfile.ui
       context: ui
+      build_args:
+        - BASE_PATH=/ui
       username: droneci
       password:
         from_secret: DRONECI_PASSWORD